Laminated body, separator, and nonaqueous secondary battery

ABSTRACT

A nonaqueous secondary battery separator, disposed between a cathode and an anode, includes: a porous base material containing a polyolefin as a main component; and a porous layer containing a polyvinylidene fluoride-based resin on at least one surface of the porous base material. The separator satisfies (C)/(D)≦0.13, where (C) represents the average pore diameter (μm) of the porous base material, and (D) represents the porosity of the porous base material, in the porous layer after being immersed for 24 hours in an electrolyte solution having a temperature of 25° C. in which electrolyte solution LiPF 6  having a concentration of 1.0 mole per liter is dissolved in a mixed solvent containing ethyl methyl carbonate, diethyl carbonate, and ethylene carbonate at a volume ratio of 50:20:30, the resin having absorbed the electrolyte solution having a volume of 0.05 to 5.00 cm 3  per square meter of the porous layer.

TECHNICAL FIELD

First, the present invention relates to a laminated body and anonaqueous electrolyte secondary battery separator including thelaminated body.

Second, the present invention relates to a laminated body, a nonaqueouselectrolyte secondary battery member, and a nonaqueous electrolytesecondary battery.

Third, the present invention relates to a laminated body and a separatorand to use of the laminated body and the separator. More specifically,the present invention relates to (i) a laminated body to be used for anonaqueous electrolyte secondary battery separator, (ii) a separatorincluding the laminated body, and (iii) a nonaqueous secondary batteryincluding the separator.

Fourth, the present invention relates to a nonaqueous secondary batteryseparator, a laminated body, a method for producing a laminated body,and a nonaqueous secondary battery.

BACKGROUND ART

First, nonaqueous electrolyte secondary batteries such as a lithiumsecondary battery are currently in wide use as batteries for devicessuch as a personal computer, a mobile telephone, and a portableinformation terminal.

A nonaqueous electrolyte secondary battery, typified by a lithiumsecondary battery, has a high energy density and may thus let a largecurrent flow and generate heat in a case where a breakage in the batteryor in the device using that battery has caused an internal or externalshort circuit. This risk has created a demand that a nonaqueouselectrolyte secondary battery should prevent more than a certain levelof heat generation to ensure a high level of safety.

Safety of a nonaqueous electrolyte secondary battery is typicallyensured by imparting to the nonaqueous electrolyte secondary battery ashutdown function, that is, a function of, in a case where there hasbeen abnormal heat generation, blocking passage of ions between thecathode and the anode with use of a separator to prevent further heatgeneration. More specifically, a nonaqueous electrolyte secondarybattery typically includes, between the cathode and the anode, aseparator that has a function of, in a case where, for example, aninternal short circuit between the cathode and the anode has caused anabnormal current to flow through the battery, block that current andprevent the flow of an excessively large current (shutdown) forprevention of further heat generation. The shutdown is performed suchthat in a case where a nonaqueous electrolyte secondary battery has beenheated to a temperature over the normal operating temperature, the heatmelts the separator, thereby clogging the pores present in theseparator. The separator preferably (i) remains unbroken by heat even ina case where the temperature inside the battery has been raised to ahigh temperature after the shutdown and (ii) maintains the shutdownstate.

The separator is typically a porous film that contains a polyolefin as amain component and that melts at, for example, approximately 80° C. to180° C. in a case of abnormal heat generation. A porous film containinga polyolefin as a main component is, however, unable to maintain a filmstructure at high temperatures not lower than the melting point of thepolyolefin, and thus breaks. This lets the cathode and the anode of thebattery be in direct contact with each other, possibly leading to ashort circuit. Further, a porous film containing a polyolefin as a maincomponent adheres poorly to an electrode. This may eventually decreasethe battery capacity and/or degrade the cycle characteristic.

There has been a separator that, in order to prevent a short circuitmentioned above, includes (i) a porous film containing a polyolefin as amain component and (ii) on at least one surface of the porous film, aheat-resistant layer including various resins and fillers.

There has also been a separator that, in order to improve theadhesiveness of the separator to an electrode, includes (i) a porousfilm containing a polyolefin as a main component and (ii) on at leastone surface of the porous film, a porous layer (adhesive layer)containing a polyvinylidene fluoride-based resin.

There has been proposed a separator including (i) a porous filmcontaining a polyolefin as a main component and (ii) a heat-resistantlayer formed excellently as a result of adjusting the wettability(critical surface tension) of the porous film and the wettability(critical surface tension) of the heat-resistant layer (PatentLiterature 1).

There has also been proposed a separator that, in order to improve theadhesiveness between a heat-resistant layer and a porous film containinga polyolefin as a main component and to improve the adhesiveness betweenfine particles included in the heat-resistant layer, contains an organicbinder (for example, a polyvinylidene fluoride-based resin) in theheat-resistant layer (Patent Literature 1).

In addition, it has been publicly known that, for example, the surfacewettability of a separator, that is, the liquid injection easiness foran electrolyte solution during battery assembly, is improved byperforming a corona treatment on a surface of the separator, that is, aporous layer mentioned above containing a resin, to introduce a polarfunctional group into the surface of the porous layer containing aresin.

Second, nonaqueous electrolyte secondary batteries, typified by alithium ion secondary battery, have a high energy density, and are thuscurrently in wide use as batteries for devices such as a personalcomputer, a mobile telephone, and a portable information terminal.

To improve characteristics such as safety of a nonaqueous electrolytesecondary battery, there have been tried various modifications to theseparator disposed between the cathode and the anode. A porous filmcontaining a polyolefin, in particular, excels in electrical insulationand exhibits good ion permeability. Such a porous film is in wide use asa separator for a nonaqueous electrolyte secondary battery. There havebeen made various proposals about such a separator.

Patent Literature 2 discloses a polyolefin-based resin cross-linkedfoamed product containing a polyolefin-based resin composition preparedby mixing alkenyl sulfonate metal salt and a foaming agent with apolyolefin-based resin. The polyolefin-based resin is crosslinked withelectron beams, and has closed cells.

Patent Literature 3 discloses a laminated microporous film including (i)a first microporous film containing a first resin composition and (ii) asecond microporous film containing a second resin composition having amelting point lower than that of the first resin composition. Thelaminated microporous film has a porosity of 50 to 70%.

If the separator is damaged during, for example, an operation ofremoving a coil wound core from an electrode group during the batteryproduction, the battery will be unable to maintain electronic insulationbetween the cathode and the anode, which will cause a batteryperformance defect, with the result of a decrease in productivity of thebattery assembly. To detect such defects in advance, battery productiontypically involves a current leak inspection before injection of anelectrolyte solution.

Patent Literature 4 discloses a separator including (i) a heat-resistantporous film containing a heat-resistant resin for reducing the defectrate during the leak inspection, (ii) a first polyolefin porous filmcovering the entire surface of the heat-resistant porous film on thecathode side, and (iii) a second polyolefin porous film covering theentire surface of the heat-resistant porous film on the anode side. Theheat-resistant resin has a melting point or heat distortion temperaturehigher than the melting point or heat distortion temperature of apolyolefin contained in the first and second polyolefin porous films.

Third, nonaqueous electrolyte secondary batteries such as a lithiumsecondary battery are currently in wide use as batteries for devicessuch as a personal computer, a mobile telephone, and a portableinformation terminal.

A nonaqueous electrolyte secondary battery, typified by a lithiumsecondary battery, has a high energy density and may thus let a largecurrent flow and generate heat in a case where a breakage in the batteryor in the device using the battery has caused an internal or externalshort circuit. This risk has created a demand that a nonaqueouselectrolyte secondary battery should prevent more than a certain levelof heat generation to ensure a high level of safety.

Safety of a nonaqueous electrolyte secondary battery is typicallyensured by imparting to the nonaqueous electrolyte secondary battery ashutdown function, that is, a function of, in a case where there hasbeen abnormal heat generation, preventing passage of ions between thecathode and the anode with use of a separator to prevent further heatgeneration. More specifically, a nonaqueous electrolyte secondarybattery typically includes, between the cathode and the anode, aseparator that has a function of, in a case where, for example, aninternal short circuit between the cathode and the anode has caused anabnormal current to flow through the battery, prevent that current andprevent the flow of an excessively large current (shutdown) forprevention of further heat generation. The shutdown is performed suchthat in a case where a nonaqueous electrolyte secondary battery has beenheated to a temperature over the normal operating temperature, the heatmelts the separator, thereby clogging the pores present in theseparator. The separator preferably (i) remains unbroken by heat even ina case where the temperature inside the battery has been raised to ahigh temperature after the shutdown and (ii) maintains the shutdownstate.

The separator is typically a porous film that contains a polyolefin as amain component and that melts at, for example, approximately 80 to 180°C. in a case of abnormal heat generation. However, a separator, which issuch a porous film, has insufficient shape stability at hightemperatures. This poses a risk that even in a case where the shutdownfunction is performed, the occurrence of contraction, breakage or thelike of the film may cause the cathode and the anode to be in direct orindirect contact with each other, leading to an internal short circuit.Specifically, a separator, which is such a porous film, may not be ableto sufficiently prevent abnormal heat generation which is caused by aninternal short circuit. This risk has created a demand for separatorsthat are capable of ensuring a high level of safety.

Patent Literature 5 proposes, as a porous film having excellent heatresistance, a porous film configured by, for example, laminating, on apolyolefin microporous film, a heat-resistant porous layer which is madeof aromatic polymer such as aromatic aramid.

Along with enlargement of lithium secondary batteries, curls ofseparators are becoming increasingly evident. In a case where a curloccurs to a separator, handling during production of a battery becomespoor. This may pose problems such as defective winding and assemblingfailure during the production of the battery. Patent Literature 6, forexample, proposes, as a technique for solving the problem, a techniquein which a multilayer porous film is prevented from thermal shrinkageand has curl-resistant properties even in a high-temperatureenvironment, the multilayer porous film being obtained by use of aporous layer-forming coating liquid which contains a multilayer porousfilm copolymer composition and inorganic particles, the mnultilayerporous film copolymer composition containing a copolymer obtained bycopolymerization of a monomer composition containing certain monomers,wherein: the monomer composition contains an unsaturated carboxylic acidmonomer at a proportion of less than 1.0% by mass; and Tg of thecopolymer, which Tg is calculated based on monomers other than acrosslinkable monomer, is −25° C. or lower.

Fourth, nonaqueous electrolyte secondary batteries (hereinafter referredto as “nonaqueous secondary battery”) such as a lithium secondarybattery are currently in wide use as batteries for devices such as apersonal computer, a mobile telephone, and a portable informationterminal.

A nonaqueous electrolyte secondary battery, typified by a lithiumsecondary battery, has a high energy density and may thus let a largecurrent flow and generate heat in a case where a breakage in the batteryor in the device using the battery has caused an internal or externalshort circuit. This risk has created a demand that a nonaqueoussecondary battery should prevent more than a certain level of heatgeneration to ensure a high level of safety.

Safety of a nonaqueous secondary battery is typically ensured byimparting to the nonaqueous secondary battery a shutdown function, thatis, a function of, in a case where there has been abnormal heatgeneration, preventing passage of ions between the cathode and the anodewith use of a separator to prevent further heat generation. Morespecifically, a nonaqueous secondary battery typically includes, betweenthe cathode and the anode, a separator that has a function of, in a casewhere, for example, an internal short circuit between the cathode andthe anode has caused an abnormal current to flow through the battery,prevent that current and prevent the flow of an excessively largecurrent (shutdown) for prevention of further heat generation. Theshutdown is performed such that in a case where a nonaqueous secondarybattery has been heated to a temperature over the normal, operatingtemperature, the heat melts the separator, thereby clogging the porespresent in the separator. The separator preferably (i) remains unbrokenby heat even in a case where the temperature inside the battery has beenraised to a high temperature after the shutdown and (ii) maintains theshutdown state.

The separator is typically made of a filmy porous base material whosemain component is, for example, a polyolefin which melts atapproximately 80 to 180° C. when abnormal heat generation occurs.However, there is a possibility that the porous base material containinga polyolefin as a main component cannot maintain a film structure at ahigh temperature which is equal to or greater than the melting point ofa polyolefin and is broken, resulting in direct contact between thecathode and the anode of a battery and so in short-circuit. Furthermore,there is a possibility that since the porous film containing apolyolefin as a main component has poor adhesion property with respectto electrodes, decrease in battery capacity and decrease in cyclecharacteristics occur.

In order to prevent occurrence of the short-circuit and to improveadhesion property of the separator with respect to the electrode, therehas been developed a separator in which a porous layer (adhesive layer)containing polyvinylidene fluoride-based resin is laminated on at leastone surface of a porous base material containing a polyolefin as a maincomponent.

For example, Patent Literature 7 discloses that a porosity of a porouslayer containing polyvinylidene fluoride-based resin is set to 30through 60% in consideration of adhesion to electrodes and ionpermeability.

CITATION LIST Patent Literature 1

PCT International Publication No. 201/129169 (Publication Date: Oct. 20,2011)

Patent Literature 2

Japanese Patent Application Publication, Tokukai, No. 2000-1561 A(Publication Date: Jan. 7, 2000)

Patent Literature 3

Japanese Patent Application Publication, Tokukai, No. 2013-28099 A(Publication Date: Feb. 7, 2013)

Patent Literature 4

PCT International Publication No. 2011/013300 Pamphlet (PublicationDate: Feb. 3, 2011)

Patent Literature 5

Japanese Patent Application Publication, Tokukai, No. 2009-205959 A

Patent Literature 6

Japanese Patent Application Publication, Tokukai, No. 2012-221889 A

Patent Literature 7

Japanese Patent No. 5432417 (Publication Date: Mar. 5, 2014)

SUMMARY OF INVENTION Technical Problem

Regarding the first point about the background art described above,conventional separators have a large difference between (i) the criticalsurface tension of a porous layer mentioned above (that is, aheat-resistant layer or an adhesive layer) containing a resin and (ii)the critical surface tension of a base material (that is, a porous filmmentioned above), and thus have a large difference between the above twolayers in terms of the liquid injection resistance for an electrolytesolution. Conventional separators are therefore problematic in that theseparator in its entirety has poor liquid injection easiness for anelectrolyte solution during assembly of a nonaqueous electrolytesecondary battery.

This leads to such problems as a lengthened step of injecting anelectrolyte solution into a battery during assembly of a nonaqueouselectrolyte secondary battery.

Regarding the second point about the background art described above,there have been developed various separators as described above. Theseseparators are, however, not perfect for the purpose of achieving goodbattery characteristics. In particular, it is necessary to furtherreduce the rate of defects such as a current leak. In view of this, theinventors of the present invention consider it necessary to produce anew secondary battery separator having a high dielectric strength, andare thus conducting research to produce a separator having a dielectricstrength higher than conventional.

The present invention has been made as a result of such research. It isan object of the present invention to provide a laminated body having ahigher dielectric strength and suitable as a separator.

Regarding the third point about the background technique, variousseparators have been developed as described above. However, suchseparators are inadequate for obtaining good battery characteristics. Inparticular, there is still room for improvement in the technique forprevention of curls.

The present invention is accomplished in view of the foregoing problem,and its object is to provide (i) a laminated body in which theoccurrence of a curl is prevented and which is to be used for aseparator and (ii) a technique for using the laminated body.

As regards the fourth point of the background art, in the technique ofPatent Literature 7, only a hole ratio of the porous layer itself isconsidered, and a state where the porous layer is built in a nonaqueoussecondary battery is not considered.

In a case where a porous layer containing polyvinylidene fluoride-basedresin is built in a nonaqueous secondary battery, the porous layer isgelatinized by an electrolyte solution, so that the porous layer canhave higher adhesion to electrodes. However, there is a possibility thatgelatinization of the porous layer causes a decrease in mobility ofions, resulting in a decrease in cycle characteristics. Specifically,there is a possibility that the decrease in ion mobility causes anincrease in charging time (particularly time for charging per constantvoltage), resulting in inconvenience such as oxidation and decompositionof an electrolyte at the cathode and deposition of a metal at the anode,which leads to a decrease in capacitance.

The present invention was made in view of the foregoing problems. Anobject of the present invention is to provide: a nonaqueous secondarybattery separator which subdues a decrease in cycle characteristics of anonaqueous secondary battery while maintaining adhesion to electrodes; alaminated body; a method for producing the laminated body; and anonaqueous secondary battery.

Solution to Problem Aspect 1 of Present Invention

Regarding the first object above, the present invention has beenaccomplished to solve the above problem, and provides a laminated bodyincluding: a porous film containing a polyolefin as a main component;and a porous layer on at least one surface of the porous film, theporous layer containing a resin, the laminated body satisfying Formula(1) below,

0≦(A)−(B)≦20 mN/m  (1),

where (A) represents a critical surface tension over an outermostsurface of the porous layer, and (B) represents a critical surfacetension that, in a case where the porous layer has been peeled from thelaminated body at an interface with the porous film, the porous film hason a side of the interface.

The laminated body may preferably be arranged such that the laminatedbody satisfies Formula (2) below,

(C)/(D)≦0.13  (2).

In Formula (2), (C) represents an average pore diameter of the porousfilm, and (D) represents a porosity of the porous film, the average porediameter (C) having a value in μm indicative of a mean value ofrespective sizes of pores in the porous film, the porosity (D) having avalue indicative of a volume proportion of void in the actual porousfilm with reference to a volume of the porous film assumed to have novoid.

The laminated body may preferably be arranged such that the resincontained in the porous layer is a polyvinylidene fluoride-based resin.

The present invention, in order to solve the above problem, furtherprovides a separator for a nonaqueous electrolyte secondary battery, theseparator including the laminated body.

The present invention, in order to solve the above problem, furtherprovides a nonaqueous electrolyte secondary battery including theseparator for a nonaqueous electrolyte secondary battery.

Aspect 2 of Present Invention

Regarding the second object above, the inventors of the presentinvention have particularly studied the amount of an increase in thedielectric strength of each layer in a separator which increase occurstogether with an increase in the content of resin per unit area of thelayer. The inventors have thus discovered that in a case where a porouslayer on one or both surfaces of a porous film contains a resin suchthat the amount of an increase in the dielectric strength of the porouslayer is not smaller than the amount of an increase in the dielectricstrength of the porous film, it is possible to produce a laminated bodyhaving a higher dielectric strength. The inventors have, as a, result,completed the present invention.

Specifically, in order to solve the above problem, a laminated body ofthe present invention is a laminated body including: a porous filmcontaining a polyolefin as a main component; and a porous layer on oneor both surfaces of the porous film, the porous layer containing aresin, the laminated body satisfying Formula (1) below,

(A)>(B)  (1),

where (A) represents an amount (V·m²/g) of an increase in a dielectricstrength of the porous layer with respect to an amount of an increase inan amount (g/m²⁾ of the resin contained per unit area of the porouslayer, and (B) represents an amount (V·m²/g) of an increase in adielectric strength of the porous film with respect to an amount of anincrease in an amount (g/m²) of the polyolefin contained per unit areaof the porous film, the laminated body satisfying Formula (2) below,

(C)/(D)≦0.13  (2).

In Formula (2) above, (C) represents an average pore diameter of theporous film, and (D) represents a porosity of the porous film, theaverage pore diameter (C) having a value in μm indicative of a meanvalue of respective sizes of pores in the porous film, the porosity (D)having a value indicative of a volume proportion of void in the actualporous film with reference to a volume of the porous film assumed tohave no void.

The laminated body of the present invention may preferably be arrangedsuch that the laminated body further satisfies (A)>2×(B),

The laminated body of the present invention may preferably be arrangedsuch that the resin is a polyvinylidene fluoride.

The laminated body of the present invention may preferably be arrangedsuch that the resin is an aromatic polyamide.

A separator of the present invention for a nonaqueous electrolytesecondary battery includes the laminated body.

A member of the present invention for a nonaqueous electrolyte secondarybattery includes in sequence: a cathode; the separator of the presentinvention for a nonaqueous electrolyte secondary battery; and an anode.

A nonaqueous electrolyte secondary battery of the present inventionincludes the separator of the present invention for a nonaqueouselectrolyte secondary battery.

Aspect 3 of Present Invention

Regarding the third object above, the inventors of the present inventionhave diligently studied the object to discover that (A) in a laminatedbody for a separator, its moisture absorption property is closelyrelated to occurrence of a curl, that (B) in a case where a laminatedbody has a moisture absorption property within a particular range,occurrence of a curl can be prevented, that (C) in a second porous layerof a laminated body for a separator, its shape is closely related tooccurrence of a curl, and that (D) in a case where a second porous layerhas a particular shape, occurrence of a curl can be prevented. Theinventors have, as a result, completed the present invention.Specifically, the present invention may be construed as any of thefollowing inventions:

(1) A laminated body including: a stack of (i) a first porous layercontaining a polyolefin-based resin and (ii) a second porous layer, adifference being 1000 ppm or less between (A) a water content rate ofthe laminated body in an atmosphere having a dew point of 20° C. and (B)a water content rate of the laminated body in an atmosphere having a dewpoint of −30° C., respective openings of pores each having an area of0.5 μm² or more occupying 30% or less of a surface of the second porouslayer.

(2) The laminated body according to (1), wherein a difference between(C) a water content of the first porous layer and (D) a water content ofthe second porous layer both in an atmosphere having a dew point of 20°C. is 10 mg/m² or less.

(3) The laminated body according to (1) or (2), wherein the respectiveopenings occupy 5% or less of the surface of the second porous layer.

(4) The laminated body according to any one of (1) to (3), wherein thesecond porous layer either (i) has a structure in which skeletons eachhaving a diameter of 1 μm or less are connected to each other to form athree-dimensional network or (ii) contains a fine resin particle.

(5) The laminated body according to any one of (1) to (4), wherein thesecond porous layer contains a polyvinylidene fluoride-based resin.

(6) The laminated body according to (4), wherein the fine resin particleis of a resin having a structure unit derived from α-olefin having 2 to4 carbon atoms.

(7) The laminated body according to any one of (1) to (6), wherein thedifference is 100 ppm or more between the water content rate (A) and thewater content rate (B).

(8) The laminated body according to any one of (1) to (7), wherein thedifference between the water content (C) and the water content (D) is 1mg/m² or more.

(9) A separator including a laminated body according to any one of (1)to (8).

(10) A nonaqueous secondary battery including a separator according to(9).

Aspect 4 of Present Invention

Regarding the fourth object above, in order to solve the above problem,a separator of the present invention for a nonaqueous secondary batteryis a separator for a nonaqueous secondary battery which separator isdisposed between a cathode and an anode both for a nonaqueous secondarybattery, the separator including: a porous base material containing apolyolefin as a main component; and a porous layer on at least onesurface of the porous base material, the porous layer containing apolyvinylidene fluoride-based resin, the separator satisfying

(C)/(D)≦0.13,

where (C) represents an average pore diameter of the porous basematerial, the average pore diameter (C) having a value in μm indicativeof a mean value of respective sizes of pores in the porous basematerial, and (D) represents a porosity of the porous base material, theporosity (D) having a value indicative of a volume proportion of void inthe actual porous base material with reference to a volume of the porousbase material assumed to have no void, in the porous layer after beingimmersed for 24 hours in an electrolyte solution having a temperature of25° C. in which electrolyte solution LiPF₆ having a concentration of 1.0mole per liter is dissolved in a mixed solvent containing ethyl methylcarbonate, diethyl carbonate, and ethylene carbonate at a volume ratioof 50:20:30, the resin having absorbed the electrolyte solution having avolume of 0.05 to 5.00 cm³ per square meter of the porous layer.

The separator of the present invention for a nonaqueous secondarybattery may preferably be arranged such that in the porous layer afterbeing immersed for 24 hours in the electrolyte solution, the resinhaving absorbed the electrolyte solution has a volume of 0.25 to 1.50cm³ per square meter of the porous layer.

The separator of the present invention for a nonaqueous secondarybattery may preferably be arranged such that the porous layer afterbeing immersed for 24 hours in the electrolyte solution has a porosityof 0.005 to 0.55.

The separator of the present invention for a nonaqueous secondarybattery may preferably be arranged such that the porous layer afterbeing immersed for 24 hours in the electrolyte solution has an averagepore diameter of 0.8 to 95.0 nm.

In order to solve the above problem, a laminated body of the presentinvention includes: the separator for a nonaqueous secondary battery;and an electrode sheet.

In order to solve the above problem, a method of the present inventionfor producing a laminated body includes the step of applying, to theporous base material or the electrode sheet, a solution in which theresin for the porous layer is dissolved and drying the solution applied.

In order to solve the above problem, a nonaqueous secondary battery ofthe present invention includes: the separator for a nonaqueous secondarybattery; and an electrode sheet.

Advantageous Effects of Invention Aspect 1 of Present Invention

Using the separator of the present invention advantageously improves theliquid injection easiness for an electrolyte solution during assembly ofa nonaqueous electrolyte secondary battery.

Aspect 2 of Present Invention

The present invention advantageously provides a separator for anonaqueous electrolyte secondary battery which separator has a higherdielectric strength.

Aspect 3 of Present Invention

The laminated body of the present invention, in a case where it is usedas a separator for a battery, advantageously prevents occurrence of acurl. This allows a battery being built to be handled easily, and isalso expected to improve the battery performance.

Aspect 4 of Present Invention

The present invention advantageously not only ensures adhesion between aseparator and an electrode, but also prevents degradation of the cyclecharacteristic of a nonaqueous secondary battery.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram (Aspect 1 of the present invention) schematicallyillustrating a laminated body of the present invention.

FIG. 2 is a diagram (Aspect 1 of the present invention) schematicallyillustrating a method of an electrolyte solution permeation testinvolving a laminated body of the present invention.

FIG. 3 is an image (Aspect 3 of the present invention) captured under ascanning electron microscope of a surface of a second porous layer of alaminated body of the present invention.

FIG. 4 is an image (Aspect 3 of the present invention) captured under ascanning electron microscope of a surface of a second porous layer of aconventional laminated body (Comparative Example 4).

DESCRIPTION OF EMBODIMENTS First Embodiment: Aspect 1 of PresentInvention

The inventors of the present invention have diligently conductedresearch and have discovered that in a case where a laminated bodyincluding: a porous film (which may hereinafter be referred to simply as“porous film”) containing a polyolefin as a main component; and a porouslayer on at least one surface of the porous film, the porous layercontaining a resin, is arranged such that the difference between (A) thecritical surface tension over the outermost surface of the porous layerand (B) the critical surface tension that, in a case where the porouslayer has been peeled from the laminated body at an interface with theporous film, the porous film has on the side of the interface is small,specifically the above difference (A)−(B) is 0 mN/m or more and 20 mN/mor less, a nonaqueous electrolyte secondary battery separator includingthe laminated body has good wettability, and the separator has, duringassembly of a nonaqueous electrolyte secondary battery, improved liquidinjection easiness for an electrolyte solution over conventionalseparators. The inventors have, as a result, completed the presentinvention.

The critical surface tension that the porous film has on the side of theinterface refers to the critical surface tension of the porous film(base material) remaining after the porous layer has been peeled fromthe laminated body which critical surface tension is over a surface ofthe porous film which surface is on the side on which the porous filmhad an interface with the porous layer.

The “electrolyte solution” for the present specification covers anyelectrolyte solution in general use for a nonaqueous electrolytesecondary battery.

Embodiment of Present Invention

The description below deals in detail with a first embodiment of thepresent invention. The present invention is, however, not limited tosuch an embodiment. Further, the present invention is not limited to thedescription of the arrangements below, but may be altered in variousways by a skilled person within the scope of the claims. Any embodimentbased on a proper combination of technical means disclosed in differentembodiments is also encompassed in the technical scope of the presentinvention. In the present specification, any numerical range expressedas “A to B” means “not less than A and not greater than B” unlessotherwise stated.

(Separator for Nonaqueous Electrolyte Secondary Battery)

One embodiment of the present invention is a laminated body including: aporous film; and a porous layer on at least one surface of the porousfilm, the porous layer containing a resin, the laminated body satisfyingFormula (1) below,

0≦(A)−(B)≦20 mN/m  (1),

where (A) represents a critical surface tension over an outermostsurface of the porous layer, and (B) represents a critical surfacetension that, in a case where the porous layer has been peeled from thelaminated body at an interface with the porous film, the porous film hason a side of the interface.

<Porous Film>

The porous film for the present invention serves as a base material ofthe separator. The porous film contains a polyolefin as a main componentand has inside itself a large number of pores connected to one another.The porous film allows a gas, a liquid, or the like to pass therethroughfrom one surface to the other.

The porous film contains a polyolefin at a proportion of 50% or more byvolume, preferably 90% or more by volume, more preferably 95% or more byvolume, of the entire porous film. The polyolefin preferably contains ahigh molecular weight component having a weight-average molecular weightof 5×10⁵ to 15×10⁶. The polyolefin particularly preferably contains ahigh molecular weight component having a weight-average molecular weightof 1,000,000 or more because such a high molecular weight componentincreases (i) the strength of the porous film and (ii) that of thelaminated body including the porous film.

Specific examples of a thermoplastic resin as the polyolefin includehomopolymers (for example, polyethylene, polypropylene, and polybutene)and copolymers (for example, ethylene-propylene copolymer) producedthrough (co)polymerization of a monomer such as ethylene, propylene,1-butene, 4-methyl-1-pentene, or 1-hexene. Among the above examples,polyethylene is preferable because it is able to prevent (shutdown) theflow of an excessively large current at a lower temperature. Examples ofthe polyethylene include a low-density polyethylene, a high-densitypolyethylene, a linear polyethylene (ethylene-α-olefin copolymer), andan ultra high molecular weight polyethylene having a weight-averagemolecular weight of 1,000,000 or more. Among these examples, an ultrahigh molecular weight polyethylene having a weight-average molecularweight of 1,000,000 or more is preferable.

The porous film may have a thickness selected as appropriate in view ofthe thickness of the laminated body. The porous film, however,preferably has a thickness of 4 to 40 μm, more preferably 7 to 30 μm, ina case where (i) a porous film is used as a base material and (ii) aporous layer is deposited on one or both surfaces of the porous film toproduce a laminated body.

The porous film has a weight per unit area selected as appropriate inview of the strength, thickness, weight, and handling easiness of thelaminated body. The weight per unit area is, however, normallypreferably 4 to 20 g/m², more preferably 5 to 12 g/m², in order toincrease the weight energy density and volume energy density of anonaqueous electrolyte secondary battery including the laminated body asa separator.

The porous film has an air permeability of preferably 30 to 500 sec/100mL, more preferably 50 to 300 see/100 mL, in terms of Gurley values. Aporous film having such an air permeability achieves sufficient ionpermeability in a case where the laminated body is used as a separator.

The porous film has a porosity (D) of preferably 0.2 to 0.8 (20 to 80%by volume), more preferably 0.3 to 0.75 (30 to 75% by volume), in orderto allow the separator to (i) retain a larger amount of electrolytesolution and (ii) achieve a function of reliably preventing (shutdown)the flow of an excessively large current at a lower temperature. Theporous film has pores each having a pore size of preferably 3 μm orless, more preferably 1 μm or less, in order to, in a case where thelaminated body is used as a separator, achieve sufficient ionpermeability and prevent particles from entering the cathode, the anode,or the like. Further, the porous film has pores having an average poresize (hereinafter referred to also as “average pore diameter (C)”), theaverage pore diameter (C) and porosity (D) of the porous film preferablysatisfying the relation (C)/(D)≦0.13, more preferably satisfying therelation (C)/(D)≦0.10. In the relation above, (C) represents the averagepore diameter of the porous film, and (D) represents the porosity of theporous film, the average pore diameter (C) having a value in μmindicative of a mean value of respective sizes of pores in the porousfilm, the porosity (D) having a value indicative of a volume proportionof void in the actual porous film with reference to a volume of theporous film assumed to have no void.

The average pore diameter (C) of the porous film is measured with use ofa palm porometer available from PMI Co., Ltd. (model: CFP-1500A). Themeasurement involves, as a test liquid, GalWick (product name) availablefrom PMI Co., Ltd., and is made of the following curves (i) and (ii) forthe porous film:

(i) Pressure-flow rate curve for the porous film as immersed in the testliquid

(ii) Pressure-flow rate curve, which is half the flow rate measured forthe dry porous film

The average pore diameter (C) of the porous film is calculated byFormula (3) below on the basis of the value of a pressure Pcorresponding to the point of intersection of the curves (i) and (ii).

(C)=4 cos θr/P  (3)

In Formula (3) above, (C) represents the average pore diameter (μm), rrepresents the surface tension (N/nm) of the test liquid, P representsthe above-mentioned pressure (Pa) corresponding to the point ofintersection, and θ represents the angle (°) of contact between theporous film and the test liquid.

The porosity (D) of the porous film is measured through the followingmethod: A square piece with a 10 cm side is cut out from the porousfilm. The weight W (g) and thickness E (cm) of the piece cut out arethen measured. The porosity (D) of the porous film is calculated byFormula (4) below on the basis of (i) the weight (W) and thickness (E)measured above and (ii) the true specific gravity ρ (g/cm³) of theporous film.

Porosity (D)=1−{(W/ρ)}/(10×10×E)  (4)

The average pore diameter (C) of the porous film is controlled through,for example, a method of, in a case of reducing the pore diameter, (i)uniformizing the dispersion state of a pore forming agent such as aninorganic filler or of a phase separating agent during production of theporous film, (ii) using, as a pore forming agent, an in organic fillerhaving smaller particle sizes, (iii) drawing the porous film in a statewhere the porous film contains a phase separating agent, or (iv) drawingthe porous film at a low extension magnification. The porosity (D) ofthe porous film is controlled through, for example, a method of, in acase of producing a porous film having a high porosity, (i) increasingthe amount of a pore forming agent such as an inorganic filler or of aphase separating agent relative to the resin such as a polyolefin, (ii)drawing the porous film after the phase separating agent has beenremoved, or (iii) drawing the porous film at a high extensionmagnification.

The above average pore diameter (C)/porosity (D) of the porous filmshould be a dominant factor in ease of infiltration of an electrolytesolution into the polyolefin base material of a nonaqueous electrolytesecondary battery separator including the porous film.

A decrease in the value of (C)/(D) means (i) a decrease in the averagepore diameter (C) of the porous film and/or (ii) an increase in theporosity (D) of the porous film.

A decrease in the average pore diameter (C) of the porous film shouldincrease the capillary force, which is presumed to serve as a drivingforce for introducing the electrolyte solution into pores inside thepolyolefin base material. Further, an increase in the porosity (D) ofthe porous film should decrease the volume of a portion of thepolyolefin base material which portion contains a polyolefin that cannotbe permeated by the electrolyte solution. This should be the reason whya decrease in the value of (C)/(D) described above increases the ease ofinfiltration of an electrolyte solution into the polyolefin basematerial of a nonaqueous electrolyte secondary battery separatorincluding the porous film.

Specifically, in a case where, as described above, (C)/(D)≦0.13,desirably (C)/(D)≦0.10, it can increase the ease of infiltration of anelectrolyte solution into the polyolefin base material of a nonaqueouselectrolyte secondary battery separator with the porous film so that theease of infiltration is sufficiently high for the separator to be inactual use as a nonaqueous electrolyte secondary battery separator. Thisindicates that the above-described (C)/(D) is a factor that influencesthe above-mentioned critical surface tension (B), which in turn meansthat adjusting (C)/(D) can control the range of (A)−(B) mentioned above.

Since the porous film for the present invention has pores, the averagepore diameter (C) of the porous film has a value of greater than 0,which also indicates that the above-described (C)/(D) returns a value ofgreater than 0.

The porous film may be produced through any method, and may be producedthrough, for example, a method of (i) adding a plasticizing agent to aresin such as a polyolefin to shape the polyolefin into a film and thenremoving the plasticizing agent with use of an appropriate solvent.

Specifically, in a case of, for example, producing a porous film withuse of (i) a polyolefin resin containing an ultra high molecular weightpolyethylene and (ii) a low molecular weight polyolefin having aweight-average molecular weight of 10,000 or less, such a porous filmis, in terms of production cost, preferably produced through the methodincluding steps of:

(1) kneading (i) 100 parts by weight of the ultra high molecular weightpolyethylene, (ii) 5 to 200 parts by weight of the low molecular weightpolyolefin having a weight-average molecular weight of 10,000 or less,and (iii) 100 to 400 parts by weight of an inorganic filler made ofcalcium carbonate and the like to produce a polyolefin resincomposition,

(2) shaping the polyolefin resin composition into a sheet,

then either

(3) removing the inorganic filler from the sheet produced in the step(2), and

(4) drawing the sheet, from which the inorganic filler has been removedin the step (3), to produce a porous film, or

(3′) drawing the sheet produced in the step (2), and

(4′) removing the inorganic filler from the sheet drawn in the step (3′)to produce a porous film.

The porous film may alternatively be a commercially available producthaving the above physical properties.

The porous film may be subjected to a hydrophilization treatment beforea porous layer is formed thereon, that is, before a coating solutiondescribed below is applied thereto. Performing a hydrophilizationtreatment on the porous film allows the critical surface tension (B) tobe adjusted. Specifically, in a case where the critical surface tension(B), which depends mainly on the porous film, is adjusted incorrespondence with the critical surface tension (A) measured in advanceof the outermost surface of the porous layer, a laminated body thatsatisfies the relation of Formula (1) above can be produced.

Specific examples of the hydrophilization, treatment include publiclyknown treatments such as (i) a chemical treatment involving an acid, analkali, or the like, (ii) a corona treatment, and (iii) a plasmatreatment. Among these hydrophilization treatments, a corona treatmentis preferable because it can not only hydrophilize the porous filmwithin a relatively short time period, but also hydrophilize only asurface and its vicinity of the porous film to leave the inside of theporous film unchanged in quality.

The porous film may include, as necessary, a porous layer other than theporous layer containing a resin. Examples of the other porous layerinclude publicly known porous layers such as a heat-resistant layer, anadhesive layer, and a protective layer. Specific examples include aporous layer identical in composition to a porous layer described belowcontaining a resin.

<Porous Layer>

The porous layer for the present invention is preferably aheat-resistant layer or adhesive layer on one or both surfaces of theporous film. The porous layer contains a resin that is preferably (i)insoluble in the electrolyte solution of the battery and (ii)electrochemically stable when the battery is in normal use. In a casewhere the porous layer is disposed on one surface of the porous film,the porous layer is preferably on a surface of the porous film whichsurface faces a cathode of a nonaqueous electrolyte secondary battery tobe produced, more preferably disposed on a surface of the porous filmwhich surface comes into contact with the cathode.

Specific examples of the resin include polyolefins such as polyethylene,polypropylene, polybutene, and ethylene-propylene copolymer;fluorine-containing resins such as polyvinylidene fluoride (PVDF) andpolytetrafluoroethylene; fluorine-containing rubbers such as vinylidenefluoride-hexafluoropropylene-tetrafluoroethylene copolymer andethylene-tetrafluoroethylene copolymer; aromatic polyamides; fullyaromatic polyamides (aramid resins); rubbers such as styrene-butadienecopolymer and a hydride thereof, methacrylic acid ester copolymer,acrylonitrile-acrylic acid ester copolymer, styrene-acrylic acid estercopolymer, ethylene propylene rubber, and polyvinyl acetate; resins witha melting point or glass transition temperature of 180° C. or highersuch as polyphenylene ether, polysulfone, polyether sulfone,polyphenylene sulfide, polyetherimide, polyamide imide, polyetheramide,and polyester; and water-soluble polymers such as polyvinyl alcohol,polyethyleneglycol, cellulose ether, sodium alginate, polyacrylic acid,polyacrylamide, and polymethacrylic acid.

Specific examples of the aromatic polyamides include poly(paraphenyleneterephthalamide), poly(methaphenylene isophthalamide),poly(parabenzaimide), poly(methabenzamide), poly(4,4′-benzanilideterephthalamide), poly(paraphenylene-4,4′-biphenylene dicarboxylic acidamide), poly(methaphenylene-4,4′-biphenylene dicarboxylic acid amide),poly(paraphenylene-2,6-naphthalene dicarboxylic acid amide),poly(methaphenylene-2,6-naphthalene dicarboxylic acid amide),poly(2-chloroparaphenylene terephthalamide), paraphenyleneterephthalamide/2,6-dichloroparaphenylene terephthalamide copolymer, andmethaphenylene terephthalamide/2,6-dichloroparaphenylene terephthalamidecopolymer. Among these, poly(paraphenylene terephthalamide) ispreferable.

Among the above resins, fluorine-containing resins and aromaticpolyamides are preferable. Among the fluorine-containing resins, apolyvinylidene fluoride-based resin is more preferable such aspolyvinylidene fluoride (PVDF) and a copolymer of vinylidene fluoride(VDF) and hexafluoropropylene (HFP). Of the two, PVDF is morepreferable.

The porous layer may contain a filler.

The porous layer for the present invention may contain a filler made oforganic matter or a filler made of inorganic matter. Specific examplesof the filler made of organic matter include fillers made of (i) ahomopolymer of a monomer such as styrene, vinyl ketone, acrylonitrile,methyl methacrylate, ethyl methacrylate, glycidyl methacrylate, glycidylacrylate, or methyl methacrylate, or (ii) a copolymer of two or more ofsuch monomers; fluorine-containing resins such aspolytetrafluoroethylene, ethylene tetrafluoride-propylene hexafluoridecopolymer, tetrafluoroethylene-ethylene copolymer, and polyvinylidenefluoride; melamine resin; urea resin; polyethylene; polypropylene; andpolyacrylic acid and polymethacrylic acid. Specific examples of thefiller made of inorganic matter include fillers made of calciumcarbonate, talc, clay, kaolin, silica, hydrotalcite, diatomaceous earth,magnesium carbonate, barium carbonate, calcium sulfate, magnesiumsulfate, barium sulfate, aluminum hydroxide, magnesium hydroxide,calcium oxide, magnesium oxide, titanium oxide, titanium nitride,alumina (aluminum oxide), aluminum nitride, mica, zeolite, or glass. Theporous layer may contain (i) only one kind of filler or (ii) two or morekinds of fillers in combination.

Among the above fillers, a filler made of inorganic matter (typicallycalled “filling material”) is suitable. A filler made of an inorganicoxide such as silica, calcium oxide, magnesium oxide, titanium oxide,alumina, mica, or zeolite is preferable. A filler made of at least onekind selected from the group consisting of silica, magnesium oxide,titanium oxide, and alumina is more preferable. A filler made of aluminais particularly preferable. While alumina has many crystal forms such asα-alumina, β-alumina, γ-alumina, and θ-alumina, any of the crystal formscan be used suitably. Among the above crystal forms, α-alumina is themost preferable because it is particularly high in thermal stability andchemical stability.

The present invention normally involves dissolving the resin in asolvent and dispersing the filler in the solution as necessary toprepare a coating solution for forming a porous layer.

The critical surface tension for the present invention has a valuedefined as indicative of a surface tension of a substance for a case inwhich a droplet has been dropped on a surface of the substance at acontact angle of 0°. The critical surface tension serves as an indicatorof the wettability of the substance. A smaller value of the criticalsurface tension means greater wettability of the substance, normallyindicating greater liquid absorbency. For a laminated body, the liquidabsorption resistance between different layers greatly influences theliquid absorbency. The critical surface tension in the presentspecification is measured through a method conforming to JIS K 6768.

Specifically, the measurement is made at 25° C. of respective contactangles θ of pure water and propylene carbonate as solvents with respectto the outermost surface of the porous layer (coating layer) of alaminated body. The measurement involves use of Drop Master 500, acontact angle measuring device available from Kyowa Interface ScienceCo., Ltd. The critical surface tension (A) is calculated on the basis ofa Zisman plot of the measurement results. Another calculation is madethrough a similar method of the critical surface tension (B) over asurface of the porous film (base material) remaining after the porouslayer (coating layer) has been peeled from the laminated body from whichsurface the porous layer (coating layer) has been peeled.

In a case where (i) the absolute value is smaller of the differencebetween the critical surface tension (A) over the outermost surface ofthe porous layer and the critical surface tension (B) of the porous filmthat, in a case where the porous layer has been peeled from thelaminated body at an interface with the porous film, the porous film hason the side of the interface and (ii) subtracting the value of thecritical surface tension (B) from the value of the critical surfacetension (A) returns a value of 0 or greater, the difference in liquidabsorption resistance is reduced between the layer of the porous filmand the porous layer, with the result of higher liquid absorbency forthe entire separator. Specifically, 0 mN/m≦(A)−(B)≦20 mN/m, desirably 4mN/m≦(A)−(B)≦19 mN/m, more desirably 4 mN/m≦(A)−(B)≦15 mN/m. Such adifference preferably achieves sufficiently high liquid absorbency ofthe entire separator for an electrolyte solution.

(Time Period of Permeation of Electrolyte Solution Through Porous Layer)

In order to reduce, to a practical level, the internal resistance of anonaqueous electrolyte secondary battery produced to include thelaminated body of the present invention as a nonaqueous electrolytesecondary battery separator, the present invention involves, duringassembly of a nonaqueous electrolyte secondary battery, (i) a step ofinjecting an electrolyte solution into a group typically including acathode, an anode, and a separator and (ii) an aging step of allowingthe electrolyte solution to permeate through the inside of theseparator. A method described below allows measurement of a time periodof permeation of an electrolyte solution into a laminated body(nonaqueous electrolyte secondary battery separator), which time periodis inversely correlated to a work time period (cycle time) of the abovetwo steps. The time period of permeation of an electrolyte solution intothe laminated body is thus presumed to serve as an indicator of the workspeed for the above two steps, in particular a time period of permeationof an electrolyte solution through the inside of the separator.

The time period of permeation of an electrolyte solution into thelaminated body is preferably shorter than 50 seconds, more preferablyshorter than 30 seconds. In a case where the time period of permeationof an electrolyte solution into the laminated body is within the aboverange, it is possible to shorten, to a practical level, the work timeperiod (cycle time) of a process of assembling a nonaqueous electrolytesecondary battery separator including the laminated body, in particularthe aging step of allowing the electrolyte solution to permeate throughthe inside of the separator.

(Method for Producing Nonaqueous Electrolyte Secondary BatterySeparator)

The laminated body of the present invention for use as a separator isproduced by, as illustrated in FIG. 1, forming, on a surface of a porousfilm as a base material, a porous layer containing a resin through, forexample, any one of methods (1) to (3) below.

(1) Method of (i) applying to a surface of the porous film a coatingsolution in which a resin for forming a porous layer is dissolved andthen (ii) immersing the resulting film into a deposition solvent as apoor solvent for the resin to deposit a porous layer containing theresin

(2) Method of (i) applying to a surface of the porous film a coatingsolution in which a resin for forming a porous layer is dissolved andthen (ii) making the coating solution acidic with use of low-boilingproton acid to deposit a porous layer containing the resin

(3) Method of (i) applying to a surface of the porous film a coatingsolution in which a resin for forming a porous layer is dissolved andthen (ii) evaporating the solvent in the coating solution by farinfrared heating or freeze drying to deposit a porous layer containingthe resin

The methods (1) and (2) may each further involve a step of, after aporous layer has been deposited, drying the laminated body produced.

The solvent (disperse medium) in which the resin is dissolved may be anysolvent that does not adversely influence the porous film, that allowsthe resin to be dissolved uniformly and stably, and that allows thefiller to be dispersed uniformly and stably. Specific examples of thesolvent (disperse medium) include water; lower alcohols such as methylalcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, and t-butylalcohol; acetone, toluene, xylene, hexane, N-methylpyrrolidone,N,N-dimethylacetamide, and N,N-dimethylformamide. The present embodimentmay use only one kind of solvent (disperse medium) or two or more kindsof solvents in combination. In a case where in any of the above methods,the resin for forming a porous layer is, for example, a PVDF-basedresin, the solvent in which the resin is dissolved is preferably anamide-based solvent such as N-methylpyrrolidone, more preferablyN-methylpyrrolidone.

The deposition solvent is, for example, a solvent (hereinafter referredto as “solvent X”) other than the solvent (disperse medium) in thecoating solution which solvent X is dissolvable in the solvent (dispersemedium) in the coating solution and which solvent X does not dissolvethe resin in the coating solution. The solvent (disperse medium) may beefficiently removed from the coating solution by (i) immersing into thesolvent X a porous film to which, the coating solution has been appliedto form a coating film, (ii) substituting the solvent X for the solvent(disperse medium) in the coating film on the porous film or a support,and then (iii) evaporating the solvent X. Specific examples of thedeposition solvent include water; lower alcohols such as methyl alcohol,ethyl alcohol, n-propyl alcohol, isopropyl alcohol, and t-butyl alcohol;and acetone. The present embodiment may use only one kind of depositionsolvent or two or more kinds of deposition solvents in combination. In acase where in the method (1), the resin for forming a porous layer is,for example, a PVDF-based resin, the solvent for depositing a porouslayer is preferably isopropyl alcohol or t-butyl alcohol.

Combining the solvent for dissolving a resin with the deposition solventallows adjustment of the difference between the critical surface tension(A) and the critical surface tension (B). Specifically, the propertiesof the above solvents influence and change the amount of the coatingsolution (that is, the resin) that enters the void in the porous film,which change in turn changes the critical surface tension (B), with theresult of a change in the difference between the critical surfacetension (A) and the critical surface tension (B).

In the method (2), the low-boiling proton acid is, for example,hydrochloric acid or acetic acid.

In the method (3), far infrared heating and freeze drying areadvantageous over other drying methods (such as air drying) in that therespective shapes of holes in the porous layer are not easily changeableduring the deposition.

The laminated body of the present invention for use as a separator mayalternatively be produced by forming, on a surface of a porous film as abase material, a porous layer containing a resin through the method (4)below.

(4) Method of (i) applying to a base material a coating solutioncontaining a disperse medium such as water and fine particles of theresin for forming a porous layer which fine particles are dispersed inthe disperse medium and (ii) drying the disperse medium for removal toform a porous layer

In the method (4), the disperse medium is preferably water. Further, thelaminated body before the drying may be immersed in a lower alcohol todilute or substitute the disperse medium such as water with the loweralcohol. In this case, the lower alcohol is preferably isopropyl alcoholor t-butyl alcohol.

In a case of producing a laminated body further including aheat-resistant layer, such a heat-resistant layer may be depositedthrough a method similar to the above method except that the resin forforming a porous layer is replaced with a resin for forming aheat-resistant layer.

To form a porous layer containing a filler, the filler may be dispersedin the coating solution in which the resin for forming a porous layer isdissolved.

There is no particular limit to how the coating solution is applied tothe porous film, that is, how a porous layer is formed on a surface of aporous film that has been subjected to a hydrophilization treatment asnecessary. In a case where a porous layer is deposited on each of bothsurfaces of the porous film, (i) a sequential deposition method may beused, which forms a porous layer on one surface of the porous film andthen forms another porous layer on the other surface, or (ii) a,simultaneous deposition method may be used, which forms two porouslayers simultaneously on respective surfaces of the porous film.

The thickness of the porous layer may be controlled by adjusting, forexample, (i) the thickness of a coating film in a wet state after thecoating, (ii) the weight ratio of the resin and the filler, and/or (iii)the solid content concentration of the coating solution (that is, thesum of the resin concentration and the filler concentration).

The coating solution is applied to the porous film through any methodthat allows the coating solution to be applied in a necessary weight perunit area with a necessary coating area. The coating solution may beapplied through a conventionally publicly known method. Specificexamples of the method include gravure coater method, small-diametergravure coater method, reverse roll coater method, transfer roll coatermethod, kiss coater method, dip coater method, knife coater method, airdoctor blade coater method, blade coater method, rod coater method,squeeze coater method, cast coater method, bar coater method, die coatermethod, screen printing method, and spray applying method.

The above drying may be performed with use of a normal drying device.The drying is performed at a drying temperature that does not decreasethe air permeability of the porous film, specifically 10° C. to 120° C.,preferably 20° C. to 80° C., to prevent pores in the porous film fromcontracting to decrease the air permeability of the porous film.

The thickness of the porous layer formed through the above method may beselected as appropriate in view of the thickness of the laminated body.In a case where (i) a porous film is used as a base material and (ii) aporous layer is deposited on one or both surfaces of the porous film toproduce a laminated body, the thickness of the porous layer ispreferably 0.1 to 20 μm (combined value in a case where a porous layeris deposited on each of both surfaces), more preferably 2 to 15 μm. Ifthe thickness of the porous layer is larger than the above range, anonaqueous electrolyte secondary battery including the laminated body asa separator may have a degraded load characteristic. If the thickness ofthe porous layer is smaller than the above range, heat generated in thebattery by an accident or the like may let the porous layer break due toa failure to resist thermal shrinkage of the porous film, with theresult of the separator being contracted.

The porous layer has a weight per unit area selected as appropriate inview of the strength, thickness, weight, and handling easiness of thelaminated body. The weight per unit area is, however, normallypreferably 0.1 to 5 g/m², more preferably 0.5 to 3 g/m², in order toincrease the weight energy density and volume energy density of anonaqueous electrolyte secondary battery including the laminated body asa separator. If the weight per unit area of the porous layer is largerthan the above range, a nonaqueous electrolyte secondary batteryincluding the laminated body as a separator will be heavy.

The porous layer has a porosity of preferably 0.1 to 0.9 (10 to 90% byvolume), more preferably 0.3 to 0.8 (30 to 80% by volume), in order toachieve sufficient ion permeability. The porous layer has pores eachhaving a pore size of preferably 3 μm or less, more preferably 1 μm orless, in order to, in a case where the laminated body is used as aseparator, achieve sufficient ion permeability and prevent particlesfrom entering the cathode, the anode, or the like. The average pore size(average pore diameter (C)) of pores in the porous film and the porosity(D) of the porous film have a relation of preferably (C)/(D)<0.13, morepreferably (C)/(D)≦0.10.

A laminated body including a porous film having the above-described(C)/(D) within the above range is preferably used as a nonaqueouselectrolyte secondary battery separator. This is because such alaminated body can increase the ease of infiltration of an electrolytesolution into the polyolefin base material of a nonaqueous electrolytesecondary battery separator so that the ease of infiltration issufficiently high for the separator to be in actual use as a nonaqueouselectrolyte secondary battery separator.

The present invention is not limited to the description of theembodiments above, but may be altered in various ways by a skilledperson within the scope of the claims. Any embodiment based on a propercombination of technical means disclosed in different embodiments isalso encompassed in the technical scope of the present invention.

The present invention may also include in its scope a laminated bodyarranged as below and a nonaqueous electrolyte secondary batteryseparator arranged as below.

[1] A laminated body including:

a porous film containing a polyolefin as a main component; and

a porous layer on at least one surface of the porous film, the porouslayer containing a resin,

the laminated body satisfying Formula (1′) below,

|(A)−(B)|<10 mN/nm  (1′),

where

(A) represents a critical surface tension over an outermost surface ofthe porous layer, and

(B) represents a critical surface tension that, in a case where theporous layer has been peeled from the laminated body at an interfacewith the porous film, the porous film has on a side of the interface.

[2] The laminated body according to [1],

wherein

the resin contained in the porous layer is a polyvinylidenefluoride-based resin.

[3] A separator for a nonaqueous electrolyte secondary battery,

the separator including

the laminated body according to [1] or [2].

Second Embodiment: Aspect 2 of Present Invention

The description below deals with a second embodiment of the presentinvention. The present invention is, however, not limited to such anembodiment. Further, the present invention is not limited to thedescription of the arrangements below, but may be altered in variousways by a skilled person within the scope of the claims. Any embodimentor example based on a proper combination of technical means disclosed indifferent embodiments and examples is also encompassed in the technicalscope of the present invention. All academic and patent documents citedin the present specification are incorporated herein by reference. Inthe present specification, any numerical range expressed as “A to B”means “not less than A and not greater than B” unless otherwise stated.

<Laminated Body>

A laminated body of the present invention is a laminated body including:a porous film containing a polyolefin as a main component; and a porouslayer on one or both, surfaces of the porous film, the porous layercontaining a resin, the laminated body satisfying Formula (1) below,

(A)>(B)  (1),

where (A) represents an amount (V·m²/g) of an increase in a dielectricstrength of the porous layer with respect to an amount of an increase inan amount (g/m²) of the resin contained per unit area of the porouslayer, and (B) represents an amount (V·m²/g) of an increase in adielectric strength of the porous film with respect to an amount of anincrease in an amount (g/m²) of the polyolefin contained per unit areaof the porous film, the laminated body satisfying Formula (2) below,

(C)/(D)≦0.13  (2),

where (C) represents an average pore diameter of the porous film, and(D) represents a porosity of the porous film. Such use of a resin thatachieves a large amount of an increase in the dielectric strength withrespect to the amount of an increase of the resin allows production of aseparator having a higher dielectric strength.

In Formula (2) above, (C) represents an average pore diameter of theporous film, and (D) represents a porosity of the porous film, theaverage pore diameter (C) having a value in μm indicative of a meanvalue of respective sizes of pores in the porous film, the porosity (D)having a value indicative of a volume proportion of void in the actualporous film with reference to a volume of the porous film assumed tohave no void.

Deficient portions are detected as defective portions during a withstandvoltage test. Such a deficient portion is caused at a portion with adecrease in the withstand voltage of the coating separator. Reducingsuch a portion having a decreased withstand voltage requires the porouslayer on the film to be uniform. (C)/(D) is presumed to be a factor thatinfluences this uniformity. In a case where (C)/(D) returns a value of0.13 or less, it is possible to prevent deficient portions from beingcaused.

The respective dielectric strengths of the porous layer and the porousfilm may each be measured with use of IMP3800K, an impulse insulationtester available from Nippon Technart Inc., through the followingprocedure:

(i) Insert a laminated body as a measurement target between an uppercylinder electrode with a diameter of φ25 mm and a lower cylinderelectrode with a diameter of φ75 mm.

(ii) Store electric charge in a capacitor inside the device to apply avoltage increasing linearly from 0 V to the laminated body between theupper and lower electrodes electrically connected to the insidecapacitor.

(iii) Continue applying the voltage until a voltage drop is detected(that is, until a dielectric breakdown occurs), and measure, as adielectric breakdown voltage, the voltage at which the voltage drop wasdetected.

(iv) Plot dielectric breakdown voltages with respect to the weight perunit area of the resin in the porous layer of the laminated body, andcalculate the dielectric strengths from the inclination of a straightline as a result of linear approximation.

In another embodiment of the present invention, the nonaqueous secondarybattery separator may include, in addition to the porous layer, aheat-resistant layer containing a heat-resistant resin. Theheat-resistant layer preferably contains aramid or fine particles ofalumina.

[Porous Film]

The porous film for the present invention serves as a base material ofthe separator. The porous film contains a polyolefin as a main componentand has inside itself a large number of pores connected to one another.The porous film allows a gas, a liquid, or the like to pass therethroughfrom one surface to the other.

The polyolefin contained in the porous film may have, for example, anykind and content such that (i) about (A) and (B) above, (A)>(B) issatisfied and that (ii) about the mean value of respective sizes ofpores in the porous film (average pore diameter (C)) and the porosity(D) of the porous film, (C)/(D)≦0.13 is satisfied. In other words, thekind, content, and the like of the polyolefin may be selected asappropriate depending on, for example, the kind and content of the resinin the porous layer and the properties of the desired porous film. Thedescription below deals with specific examples each satisfying the aboverelations.

The porous film contains a polyolefin at a proportion of 50% or more byvolume, preferably 90% or more by volume, more preferably 95% or more byvolume, of the entire porous film. The polyolefin preferably contains ahigh molecular weight component having a weight-average molecular weightof 5×10⁵ to 15×10⁶. The polyolefin particularly preferably contains ahigh molecular weight component having a weight-average molecular weightof 1,000,000 or more because such a high molecular weight componentincreases (i) the strength of the porous film and (ii) that of thelaminated body including the porous film. The expression “amount of thepolyolefin contained per unit area of the porous film.” as used for thepresent invention refers to the proportion of the polyolefin in theporous film described herein.

The dielectric strength of the porous film may be measured through aconventionally publicly known method. The dielectric strength may bemeasured, for example, with use of IMP-1090, a lithium ion batteryinsulation tester available from Nippon Technart Inc., with reference tothe attached instruction manual.

Specific examples of a thermoplastic resin as the polyolefin includehomopolymers (for example, polyethylene, polypropylene, and polybutene)and copolymers (for example, ethylene-propylene copolymer) producedthrough (co)polymerization of a monomer such as ethylene, propylene,1-butene, 4-methyl-1-pentene, or 1-hexene. Among the above examples,polyethylene is preferable because it is able to prevent (shutdown) theflow of an excessively large current at a lower temperature. Examples ofthe polyethylene include a low-density polyethylene, a high-densitypolyethylene, a linear polyethylene (ethylene-α-olefin copolymer), andan ultra high molecular weight polyethylene having a weight-averagemolecular weight of 1,000,000 or more. Among these examples, an ultrahigh molecular weight polyethylene having a weight-average molecularweight of 1,000,000 or more is preferable.

The porous film may have a thickness selected as appropriate in view ofthe thickness of the laminated body. The porous film, however,preferably has a thickness of 4 to 40 μm, more preferably 7 to 30 μm, ina case where (i) a porous film is used as a base material and (ii) aporous layer is deposited on one or both surfaces of the porous film toproduce a laminated body.

The porous film contains a polyolefin having a weight per unit areaselected as appropriate in view of the strength, thickness, weight, andhandling easiness of the laminated body. The weight is, however,normally preferably 4 to 20 g/m², more preferably 5 to 12 g/m², in orderto increase the weight energy density and volume energy density of anonaqueous electrolyte secondary battery including the laminated body asa separator.

The porous film has an air permeability of preferably 30 to 500 see/100mL, more preferably 50 to 300 see/100 mL, in terms of Gurley values. Aporous film having such an air permeability achieves sufficient ionpermeability in a case where the laminated body is used as a separator.

The porous film has a porosity (D) of preferably 0.2 to 0.8 (20 to 80%by volume), more preferably 0.3 to 0.75 (30 to 75% by volume), in orderto allow the separator to (i) retain a larger amount of electrolytesolution and (ii) achieve a function of reliably preventing (shutdown)the flow of an excessively large current at a lower temperature. Theporous film has pores each having a pore size of preferably 3 μm orless, more preferably 1 μm or less, in order to, in a case where thelaminated body is used as a separator, achieve sufficient ionpermeability and prevent particles from entering the cathode, the anode,or the like. Further, the porous film has pores having an average poresize (hereinafter referred to also as “average pore diameter (C)”), theaverage pore diameter (C) and porosity (D) of the porous film preferablysatisfying the relation (C)/(D)≦0.13, more preferably satisfying therelation (C)/(D)≦0.10.

The average pore diameter (C) of the porous film is measured with use ofa palm porometer available from PMI Co., Ltd. (model: CFP-1500A). Themeasurement involves, as a test liquid, GalWick (product name) availablefrom PMI Co., Ltd., and is made of the following curves (i) and (ii) forthe porous film:

(i) Pressure-flow rate curve for the porous film as immersed in the testliquid

(ii) Pressure-flow rate curve, which is half the flow rate measured forthe dry porous film

The average pore diameter (C) of the porous film is calculated byFormula (3) below on the basis of the value of a pressure Pcorresponding to the point of intersection of the curves (i) and (ii).

(C)=4 cos θr/P  (3)

In Formula (3) above, (C) represents the average pore diameter (μm), rrepresents the surface tension (N/m) of the test liquid, P representsthe above-mentioned pressure (Pa) corresponding to the point ofintersection, and θ represents the angle (°) of contact between theporous film and the test liquid.

The porosity (D) of the porous film is measured through the followingmethod: A square piece with a 10 cm side is cut out from the porousfilm. The weight W (g) and thickness E (cm) of the piece cut out arethen measured. The porosity (D) of the porous film is calculated byFormula (4) below on the basis of (i) the weight (W) and thickness (E)measured above and (ii) the true specific gravity ρ (g/cm³) of theporous film.

Porosity (D)=1−{(W/ρ)}/(10×10×E)  (4)

The average pore diameter (C) of the porous film is controlled through,for example, a method of, in a case of reducing the pore diameter, (i)uniformizing the dispersion state of a pore forming agent such as aninorganic filler or of a phase separating agent during production of theporous film, (ii) using, as a pore forming agent, an inorganic fillerhaving smaller particle sizes, (iii) drawing the porous film in a statewhere the porous film contains a phase separating agent, or (iv) drawingthe porous film at a low extension magnification. The porosity (D) ofthe porous film is controlled through, for example, a method of, in acase of producing a porous film having a high porosity, (i) increasingthe amount of a pore forming agent such as an inorganic filler or of aphase separating agent relative to the resin such as a polyolefin, (ii)drawing the porous film after the phase separating agent has beenremoved, or (iii) drawing the porous film at a high extensionmagnification.

The above average pore diameter (C)/porosity (D) of the porous film ispresumed to be a factor that influences uniformity of the porous layeron the film which uniformity is necessary to reduce a portion with adecrease in the withstand voltage of the coating separator which portioncauses a deficient portion, which is detected as a defective portionduring a withstand voltage test.

If (C)/(D) returns a value outside the above range, that is, a valuegreater than 0.13, the porous film has either a larger average porediameter (C) or a smaller porosity (D).

If the porous film has a larger average pore diameter (C), an increasein the size of holes (pores) in the porous film will make it more likelyfor the resin to enter the pores in the base material (porous film).This will generate a portion with a low withstand voltage, at which theamount of resin is locally decreased per unit area of the depositedporous layer (or no deposited resin is present in some cases). If theporous film has a smaller porosity (D), the resin will be prevented fromentering the pores in the porous film. This will prevent the laminatedbody, which includes the porous film and the porous layer, from having asufficient interface strength. There will more likely be generated aportion with a low withstand voltage, at which the deposited porouslayer is locally lost or deficient.

Thus, in the case where the above-described (C)/(D) returns a value of0.13 or less, it is possible to produce a laminated body in which (i)generation of a portion with a low withstand voltage is prevented, and(ii) the deficient portion is reduced during a withstand voltage test,the laminated body thus having a higher dielectric strength and beingsuitable as a separator.

Since the porous film for the present invention has pores, the averagepore diameter (C) of the porous film has a value of greater than 0,which also indicates that the above-described (C)/(D) returns a value ofgreater than 0.

The porous film may be produced through any method, and may be producedthrough, for example, a method of (i) adding a plasticizing agent to aresin such as a polyolefin to shape the polyolefin into a film and thenremoving the plasticizing agent with use of an appropriate solvent.

Specifically, in a case of, for example, producing a porous film withuse of (i) a polyolefin resin containing an ultra high molecular weightpolyethylene and (ii) a low molecular weight polyolefin having aweight-average molecular weight of 10,000 or less, such a porous filmis, in terms of production cost, preferably produced through the methodincluding steps of:

(1) kneading (i) 100 parts by weight of the ultra high molecular weightpolyethylene, (ii) 5 to 200 parts by weight of the low molecular weightpolyolefin having a weight-average molecular weight of 10,000 or less,and (iii) 100 to 400 parts by weight of an inorganic filler made ofcalcium carbonate and the like to produce a polyolefin resincomposition,

(2) shaping the polyolefin resin composition into a sheet,

then either

(3) removing the inorganic filler from the sheet produced in the step(2), and

(4) drawing the sheet, from which the inorganic filler has been removedin the step (3), to produce a porous film, or

(3′) drawing the sheet produced in the step (2), and

(4′) removing the inorganic filler from the sheet drawn in the step (3′)to produce a porous film.

The porous film may alternatively be a commercially available producthaving the above physical properties.

The porous film is preferably subjected to a hydrophilization treatmentbefore a porous layer is formed thereon, that is, before a coatingsolution described below is applied thereto. Performing ahydrophilization treatment on the porous film further improves thecoating easiness of the coating solution and thus allows a more uniformporous layer to be formed. This hydrophilization treatment is effectivein a case where the solvent (disperse medium) contained in the coatingsolution has a high proportion of water. Specific examples of thehydrophilization treatment include publicly known treatments such as (i)a chemical treatment involving an acid, an alkali, or the like, (ii) acorona treatment, and (iii) a plasma treatment. Among thesehydrophilization treatments, a corona treatment is preferable because itcan not only hydrophilize the porous film within a relatively short timeperiod, but also hydrophilize only a surface and its vicinity of theporous film to leave the inside of the porous film unchanged in quality.

[Porous Layer]

The porous layer for the present invention is a heat-resistant layerprovided on one or both surfaces of the porous film and containing atleast resin. The resin preferably has a three-dimensional networkstructure.

The resin contained in the porous layer is any resin such that about (A)and (B) above, (A)>(B) is satisfied. The resin is, however, preferably(i) insoluble in the electrolyte solution of the battery and (ii)electrochemically stable when the battery is in normal use. Whether theresin has a three-dimensional network structure may be examined, forexample, under a scanning electron microscope (SEM).

The “amount of the resin contained per unit area of the porous layer”for the present invention may be calculated from (i) the respectiveamounts of a resin, a monomer, a filler, and the like for forming aporous layer and (ii) the area of the porous layer produced.

The dielectric strength of the porous layer may be measured through amethod similar to the method described above for measuring thedielectric strength of the porous film.

Specific examples of the resin contained in the porous layer includepolyolefins such as polyethylene, polypropylene, polybutene, andethylene-propylene copolymer; fluorine-containing resins such aspolyvinylidene fluoride (PVDF) and polytetrafluoroethylene;fluorine-containing rubbers such as vinylidenefluoride-hexafluoropropylene-tetrafluoroethylene copolymer andethylene-tetrafluoroethylene copolymer; aromatic polyamides; fullyaromatic polyamides (aramid resins); rubbers such as styrene-butadienecopolymer and a hydride thereof, methacrylic acid ester copolymer,acrylonitrile-acrylic acid ester copolymer, styrene-acrylic acid estercopolymer, ethylene propylene rubber, and polyvinyl acetate; resins witha melting point or glass transition temperature of 180° C. or highersuch as polyphenylene ether, polysulfone, polyether sulfone,polyphenylene sulfide, polyetherimide, polyamide imide, polyetheramide,and polyester; and water-soluble polymers such as polyvinyl alcohol,polyethyleneglycol, cellulose ethers, polyacrylic acid, polyacrylamide,and polymethacrylic acid.

Specific examples of the aromatic polyamides include poly(paraphenyleneterephthalamide), poly(methaphenylene isophthalamide),poly(parabenzamide), poly(methabenzamide), poly(4,4′-benzanilideterephthalamide), poly(paraphenylene-4,4′-biphenylene dicarboxylic acidamide), poly(methaphenylene-4,4′-biphenylene dicarboxylic acid amide),poly(paraphenylene-2,6-naphthalene dicarboxylic acid amide),poly(methaphenylene-2,6-naphthalene dicarboxylic acid amide),poly(2-chloroparaphenylene terephthalamide), paraphenyleneterephthalamide/2,6-dichloroparaphenylene terephthalamide copolymer, andmethaphenylene terephthalamide/2,6-dichloroparaphenylene terephthalamidecopolymer. Among these, poly(paraphenylene terephthalamide) ispreferable.

The polyimide is preferably a fully aromatic polyimide produced throughcondensation polymerization of an aromatic diacid anhydride and adiamine. Examples of the diacid anhydride include pyromelliticdianhydride, 3,3′,4,4′-diphenyl sulfone tetracarboxylic dianhydride,3,3′,4,4′-benzophenone tetracarboxylic dianhydride,2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane, and 3,3′,4,4′-biphenyltetracarboxylic dianhydride. Examples of the diamine includeoxydianiline, paraphenylenediamine, benzophenone diamine,3,3-methylenedianiline, 3,3′-diaminobenzophenone, 3,3′-diaminodiphenylsulfone, and 1,5′-naphthalene diamine.

The polyamide imide is, for example, produced through condensationpolymerization of (i) aromatic dicarboxylic acid and aromaticdiisocyanate or (ii) aromatic diacid anhydride and aromaticdiisocyanate. Examples of the aromatic dicarboxylic acid includeisophthalic acid and terephthalic acid. Examples of the aromatic diacidanhydride include trimellitic anhydride. Examples of the aromaticdiisocyanate include 4,4′-diphenylmethane diisocyanate, 2,4-tolylenediisocyanate, 2,6-tolylene diisocyanate, ortho tolylene diisocyanate,and m-xylene diisocyanate.

Among the above resins, a polyolefin, a fluorine-containing resin, anaromatic polyamide, and a water-soluble polymer are preferable. Anaromatic polyamide and polyvinylidene fluoride are more preferable for ahigher dielectric strength.

The porous layer may contain a filler. In a case where the porous layercontains a filler, the resin functions also as a binder resin.

The porous layer for the present invention may contain a filler made oforganic matter or a filler made of inorganic matter. Specific examplesof the filler made of organic matter include fillers made of (i) ahomopolymer of a monomer such as styrene, vinyl ketone, acrylonitrile,methyl methacrylate, ethyl methacrylate, glycidyl methacrylate, glycidylacrylate, or methyl acrylate, or (ii) a copolymer of two or more of suchmonomers; fluorine-containing resins such as polytetrafluoroethylene,ethylene tetrafluoride-propylene hexafluoride copolymer,tetrafluoroethylene-ethylene copolymer, and polyvinylidene fluoride;melamine resin; urea resin; polyethylene; polypropylene; and polyacrylicacid and polymethacrylic acid. Specific examples of the filler made ofinorganic matter include fillers made of calcium carbonate, talc, clay,kaolin, silica, hydrotalcite, diatomaceous earth, magnesium carbonate,barium carbonate, calcium sulfate, magnesium sulfate, barium sulfate,aluminum hydroxide, magnesium hydroxide, calcium oxide, magnesium oxide,titanium oxide, titanium nitride, alumina (aluminum oxide), aluminumnitride, mica, zeolite, or glass. The porous layer may contain (i) onlyone kind of filler or (ii) two or more kinds of fillers in combination.

Among the above fillers, an inorganic particle is preferable. Theinorganic particle is a filler made of inorganic matter typically called“filling material”. The inorganic particle is, for example, a fillermade of an inorganic oxide such as silica, calcium oxide, magnesiumoxide, titanium oxide, alumina, mica, or zeolite is preferable. A fillermade of at least one kind selected from the group consisting of silica,magnesium oxide, titanium oxide, and alumina is more preferable. Afiller made of alumina is particularly preferable. While alumina hasmany crystal forms such as α-alumina, β-alumina, γ-alumina, andθ-alumina, any of the crystal forms can be used suitably. Among theabove crystal forms, α-alumina is the most preferable because it isparticularly high in thermal stability and chemical stability.

The filler has a shape that varies depending on, for example, (i) themethod for producing the organic matter or inorganic matter as a rawmaterial and (ii) the condition under which the filler is dispersed whenthe coating solution for forming a porous layer is prepared. The fillermay, as long as it has the particle diameter below, have any of variousshapes such as a spherical shape, an oblong shape, a rectangular shape,a gourd shape, or an indefinite, irregular shape.

In a case where (i) the porous layer contains a filler, and (ii) thefiller is an inorganic particle, the filler is contained in an amount ofpreferably 10 to 90% by weight, more preferably 25 to 75% by weight,with respect to the porous layer in its entirety. In a case where theinorganic particle is contained in an amount of 90% by weight or less,preferably 75%, by weight or less, with respect to the resin layer inits entirety, the resin layer can have improved dielectric strength. Thelaminated body of the present invention is produced through a method of,for example, (i) dissolving a resin in a solvent and dispersing theabove filler in the solution as necessary to prepare a coating solutionfor forming a porous layer and then (ii) applying the coating solutionto one or both surfaces of the porous film and drying the coatingsolution applied to form a porous layer for the present invention. Inother words, the coating solution is applied to one or both surfaces ofthe porous film and then dried to produce a laminated body of thepresent invention, which includes (i) a porous film and (ii) a porouslayer on one or both surfaces of the porous film.

The laminated body of the present invention may be produced through anyof various methods as long as the method allows production of thelaminated body described above.

In a case where the porous layer is to contain, for example, afluorine-containing resin, the laminated body of the present inventionmay be produced through a method of (i) applying to the porous film acoating solution containing a fluorine-containing resin to form acoating layer and then (ii) curing the fluorine-containing resin in thecoating layer to form a porous layer integrally on the porous film.

A porous layer containing a fluorine-containing resin may be formedthrough, for example, the wet coating method below. Forming a porouslayer through such a method allows the porous layer to have athree-dimensional network structure. First, a fluorine-containing resinis dissolved in a solvent, and as necessary, a filler is dispersed inthe solution to prepare a coating solution. This coating solution isapplied to a porous film, and then the porous film with the coatingsolution applied is immersed in an appropriate setting liquid to inducephase separation and cure the fluorine-containing resin. Performingthese steps forms, on the porous film, a layer containing afluorine-containing resin and having a porous structure (preferably athree-dimensional network structure). This layer is then washed withwater and dried to remove the setting liquid from the layer having aporous structure.

A specific example is the following method:

(Method 1)

(a) Prepare a solution in which a fluorine-containing resin is dissolvedin a solvent.

(b) Apply the solution to a porous film to form a coating film.

(c) Perform an operation such as immersing, into a solvent that does notdissolve the fluorine-containing resin, the wet coating film to separatethe fluorine-containing resin out of the coating film.

(d) As necessary, immerse the wet coating film, in which thefluorine-containing resin has been separated, again into a solvent thatdoes not dissolve the fluorine-containing resin and wash the coatingfilm.

(e) Dry the wet coating film, in which the fluorine-containing resin hasbeen separated.

The description below deals in detail with a wet coating method suitablefor the present invention.

Suitable examples of the solvent (hereinafter referred to also as “goodsolvent”) for use in the preparation of the coating solution whichsolvent dissolves a fluorine-containing resin include polar amidesolvents such as N-methyl 2-pyrrolidone (NMP), dimethylacetamide,dimethylformamide, and dimethylformamide.

To form a good porous structure, the good solvent is preferably mixedwith a phase separating agent for inducing phase separation. Examples ofthe phase separating agent include water, methanol, ethanol, propylalcohol, butyl alcohol, butandiol, ethylene glycol, propylene glycol,and tripropylene glycol. The phase separating agent is preferably addedin an amount that achieves viscosity suitable for the coating.

To form a good porous structure, the solvent is preferably a mixedsolvent containing (i) 60% or more by mass of a good solvent and (ii) 5%by mass to 40% by mass of a phase separating agent. To form a goodporous structure, the coating solution preferably contains afluorine-containing resin at a concentration of 3% by mass to 10% bymass.

To impart slidability to the porous layer and form a good porousstructure, the coating solution contains a filler at a proportion ofpreferably 1% by mass to 30% by mass, more preferably 3%, by mass to 28%by mass, with respect to the total amount of the fluorine-containingresin and the filler.

The setting liquid typically includes (i) a good solvent and phaseseparating agent for use in the preparation of the coating solution and(ii) water. It is preferable in terms of production that the goodsolvent and the phase separating agent be mixed at a ratio equal to thatof the mixed solvent for use in the dissolution of thefluorine-containing resin. For good formation of a porous structure andproductivity, the water concentration is preferably 40% by mass to 90%by mass.

The coating solution may be applied to the porous film through aconventional coating method, for example, with use of a Meyer bar, a diecoater, a reverse roll coater, or a gravure coater.

The porous layer may alternatively be produced through a dry coatingmethod instead of a wet coating method described above. The dry coatingmethod is a method of (i) applying to a porous film a coating solutioncontaining a fluorine-containing resin, a filler, and a solvent and then(ii) drying the coating layer for removal of the solvent throughvolatilization to produce a porous layer. The dry coating method,however, likely produces a closely packed coating layer as compared tothe wet coating method. Thus, to form a good porous structure(three-dimensional network structure), the wet coating method ispreferable.

The separator may alternatively be produced through a method of (i)preparing a porous layer as a separate sheet, (ii) placing the porouslayer on a porous film, and (iii) combining the porous layer with theporous film through thermocompression or with use of an adhesive. Theporous layer may be prepared as a separate sheet through a method of,for example, (i) applying to a release sheet a coating solutioncontaining a fluorine-containing resin and a filler, (ii) forming aporous layer through the wet coating method or dry coating methoddescribed above, and (iii) peeling the porous layer from the releasesheet.

In a case where the porous layer is to contain a resin other than afluorine-containing resin, a laminated body may be produced through, forexample, a method of (i) preparing a porous film and a porous layerseparately and (ii) combining the porous film and the porous layer witheach other or a method of (i) preparing a coating solution containing amedium as well as a component for a porous layer, (ii) applying thecoating solution to a porous film, and (iii) removing the medium. Thelatter of these methods is simple and preferable.

The medium is a solvent or a disperse medium, and simply needs to becapable of dissolving or dispersing a component for a porous layeruniformly and stably. Examples of the medium include water, alcoholssuch as methanol, ethanol, and isopropanol, acetone, toluene, xylene,hexane, N-methylpyrrolidone, N,N-dimethylacetamide, andN,N-dimethylformamide. The present invention may use only one of theabove mediums, or mix two or more of the above mediums with each otherfor use as long as the two or more mediums are dissolved in each other.

In terms of process or environmental load, it is preferable for themedium to contain water at 80% or more by weight, more preferablycontain only water.

The coating solution may be applied to the porous film through anymethod that allows uniform wet coating, and may be applied through aconventionally publicly known method. Examples of the application methodinclude capillary coating method, spin coating method, slit die coatingmethod, spray coating method, roll coating method, screen printingmethod, flexographic printing method, bar coater method, gravure coatermethod, and die coater method. The thickness of the porous layer may becontrolled by adjusting (i) the amount of the coating solution to beapplied, (ii) the concentration of the polymer in the coating solution,and/or (iii) in a case where the coating solution contains fineparticles, the ratio of the fine particles to the polymer.

The coating solution may be prepared through any method that allows ahomogeneous coating solution to be prepared. In a case where the coatingsolution is to contain a filler, in particular, the coating solution ispreferably prepared through a method such as mechanical stirring method,ultrasonic dispersion method, high-pressure dispersion method, or mediadispersion method, among which high-pressure dispersion method ispreferable because the method makes it easy to disperse a filler moreuniformly. The mixing order during such an operation may be any order aslong as it causes no particular problem such as generation ofprecipitate. For instance, the polymer and any other component such as afiller may be (i) added together to a medium and mixed with each other,(ii) added in any order to a medium and mixed with each other, or (iii)first dissolved or dispersed in respective mediums and then mixed witheach other.

In a case where the medium for the coating solution contains water, itis preferable to perform a hydrophilization treatment on the porous filmbefore applying the coating solution to the porous film. Performing ahydrophilization treatment on the porous film further improves theapplication property, and allows production of a more homogeneous porouslayer. A hydrophilization treatment is particularly effective in a casewhere the medium contains water at a high concentration.

Examples of the hydrophilization treatment include (i) a chemicaltreatment involving an acid, an alkali, or the like, (ii) a coronatreatment, and (iii) a plasma treatment.

Among the above hydrophilization treatments, a corona treatment ispreferable because it can not only hydrophilize the porous film within arelatively short time period, but also reform the polyolefin throughcorona discharge only at a surface and its vicinity of the porous filmand leave the inside of the porous film unchanged in quality whileensuring a high application property.

The medium is removed from the coating solution on the porous filmpreferably by drying the porous film because drying is simple. Examplesof the drying method include natural drying, blow drying, drying byheating, and drying under reduced pressure, among which drying byheating is preferable. Although depending on the medium used, the dryingtemperature is preferably 30° C. to 80° C., more preferably 50° C. to80° C. A drying temperature of 30° C. or higher allows a sufficientdrying speed. A drying temperature of 80° C. or lower allows depositionof a porous film having good appearance.

The thickness of the porous layer of the present invention formedthrough the above method may be selected as appropriate in view of thethickness of the laminated body. In a case where (i) a porous film isused as a base material and (ii) a porous layer is deposited on one orboth surfaces of the porous film to produce a laminated body, thethickness of the porous layer is preferably 0.1 to 20 μm (combined valuein a case where a porous layer is deposited on each of both surfaces),more preferably 2 to 15 μm. If the thickness of the porous layer islarger than the above range, a nonaqueous electrolyte secondary batteryincluding the laminated body as a separator may have a degraded loadcharacteristic. If the thickness of the porous layer is smaller than theabove range, heat generated in the battery by an accident or the likemay let the porous layer break due to a failure to resist thermalshrinkage of the porous film, with the result of the separator beingcontracted.

The physical properties of the porous layer are described below toindicate, in a case where a porous layer is deposited on each of bothsurfaces of a porous film, at least the physical properties of a porouslayer on a surface of the porous film which surface faces the cathodewhen the porous film is included in a nonaqueous electrolyte secondarybattery.

The porous layer has a weight per unit area selected as appropriate inview of the strength, thickness, weight, and handling easiness of thelaminated body. The weight per unit area is, however, normallypreferably 0.1 to 5 g/m², more preferably 0.5 to 3 g/m², in order toincrease the weight energy density and volume energy density of anonaqueous electrolyte secondary battery including the laminated body asa separator. If the weight per unit area of the porous layer is largerthan the above range, a nonaqueous electrolyte secondary batteryincluding the laminated body as a separator will be heavy.

Depositing a porous layer on one or both surfaces of the porous filmthrough the method described above allows the laminated body of thepresent invention to be formed. The laminated body of the presentinvention, in other words, includes (i) a porous film and (ii) theabove-described porous layer on one or both surfaces of the porous film.

The laminated body has an air permeability of preferably 30 to 800see/100 mL, more preferably 50 to 500 sec/100 mL, in terms of Gurleyvalues. A laminated body having such an air permeability achievessufficient ion permeability in a case where the laminated body is usedas a separator. An air permeability larger than the above range meansthat the laminated body has a high porosity and thus has a roughlaminated structure. This may result in the laminated body havingdecreased strength, in particular insufficient shape stability at hightemperatures. An air permeability smaller than the above range, on theother hand, may prevent the laminated body from having sufficient ionpermeability when used as a separator and thus degrade thecharacteristics of a nonaqueous electrolyte secondary battery to beproduced.

The laminated body is preferably arranged such that about (A) and (B)above, (A)>2×(B) is satisfied. In a case where (A) and (B) above satisfythis relation, the withstand voltage property is improved at a portionat which the deposited porous layer is deficient in the laminated bodyproduced. Thus, even in a case where the deposited porous layer has, forexample, a portion with a structural deficiency or decreased thickness,the present embodiment allows production of (i) a laminated body thatdoes not become defective during a withstand voltage test and (ii) anonaqueous electrolyte secondary battery separator including such alaminated body.

In a case where a withstand voltage test is conducted on a laminatedbody (that is, a voltage is applied to a laminated body), the laminatedbody preferably has 30 or less, more preferably 25 or less, deficientportions caused by the withstand voltage test. In a case where thenumber of such deficient portions is reduced to fall within the aboverange, a nonaqueous electrolyte secondary battery assembled to includethe laminated body as a separator can have a decreased rate of operationdefects.

The laminated body of the present invention may further include, inaddition to the porous film and the porous layer, a publicly knownporous film such as a heat-resistant layer, an adhesive layer, or aprotective layer as necessary as long as the present invention canattain its objects.

<Nonaqueous Electrolyte Secondary Battery>

The nonaqueous electrolyte secondary battery of the present inventionincludes a laminated body as a separator. More specifically thenonaqueous electrolyte secondary battery of the present inventionincludes a nonaqueous electrolyte secondary battery member including acathode, a laminated body, and an anode arranged in that order. Thedescription below deals with a lithium ion secondary battery as anexample of the nonaqueous electrolyte secondary battery. The constituentelements of the nonaqueous electrolyte secondary battery other than thelaminated body are not limited to the constituent elements describedbelow.

The nonaqueous electrolyte secondary battery of the present inventionmay use, for example, a nonaqueous electrolyte solution prepared bydissolving a lithium salt in an organic solvent. Examples of the lithiumsalt include LiClO₄, LiPF₆, LiAsF₆, LiSbF₆, LiBF₄, LiCF₃SO₃,LiN(CF₃SO₂)₂, LiC(CF₃SO₂)₃, Li₂B₁₀Cl₁₀, lower aliphatic carboxylic acidlithium salt, and LiAlCl₄. The present embodiment may use only one kindof the above lithium salts or two or more kinds of the above lithiumsalts in combination. The present embodiment preferably uses, among theabove lithium salts, at least one fluorine-containing lithium saltselected from the group consisting of LiPF₆, LiAsF₆, LiSbF₆, LiBF₄,LiCF₃SO₃, LiN(CF₃SO₂)₂, and LiC(CF₃SO₂)₃.

Specific examples of the organic solvent in the nonaqueous electrolytesolution include carbonates such as ethylene carbonate, propylenecarbonate, dimethyl carbonate, diethyl carbonate, ethyl methylcarbonate, 4-trifluoromethyl-1, 3-dioxolane-2-on, and 1, 2-di(methoxycarbonyloxy)ethane; ethers such as 1,2-dimethoxyethane,1,3-dimethoxypropane, pentafluoropropyl methylether,2,2,3,3-tetrafluoropropyl difluoro methylether, tetrahydrofuran, and2-methyl tetrahydrofuran; esters such as methyl formate, methyl acetate,and γ-butyrolactone; nitriles such as acetonitrile and butyronitrile;amides such as N,N-dimethylformamide and N,N-dimethylacetamide;carbamates such as 3-methyl-2-oxazolidone; sulfur-containing compoundssuch as sulfolane, dimethyl sulfoxide, and 1,3-propane sultone; andfluorine-containing organic solvents each prepared by introducing afluorine group into the organic solvent. The present embodiment may useonly one kind of the above organic solvents or two or more kinds of theabove organic solvents in combination. Among the above organic solvents,carbonates are preferable. A mixed solvent of a cyclic carbonate and anacyclic carbonate or a mixed solvent of a cyclic carbonate and an etheris more preferable. The mixed solvent of a cyclic carbonate and anacyclic carbonate is preferably a mixed solvent of ethylene carbonate,dimethyl carbonate, and ethyl methyl carbonate because such a mixedsolvent allows a wider operating temperature range, and is not easilydecomposed even in a case where the present embodiment uses, as an anodeactive material, a graphite material such as natural graphite orartificial graphite.

The cathode is normally a sheet-shaped cathode including (i) a cathodemix containing a cathode active material, a conductive material, and abinding agent and (ii) a cathode current collector supporting thecathode mix thereon.

The cathode active material is, for example, a material capable of beingdoped and dedoped with lithium ions. Specific examples of such amaterial include a lithium complex oxide containing at least onetransition metal such as V, Mn, Fe, Co, or Ni. Among such lithiumcomplex oxides, (i) a lithium complex oxide having an α-NaFeO₂ structuresuch as lithium nickelate and lithium cobaltate and (ii) a lithiumcomplex oxide having a spinel structure such as lithium manganese spinelare preferable because such lithium complex oxides have a high averagedischarge potential. The lithium complex oxide containing the at leastone transition metal may further contain any of various metallicelements, and is more preferably complex lithium nickelate. Further, thecomplex lithium nickelate particularly preferably contains at least onemetallic element selected from the group consisting of Ti, V, Cr, Mn,Fe, Co, Cu, Ag, Mg, Al, Ga, In, and Sn at a proportion of 0.1 to 20 mol% with respect to the sum of the number of moles of the at least onemetallic element and the number of moles of Ni in the lithium nickelate.This is because such a complex lithium nickelate allows an excellentcycle characteristic for use in a high-capacity battery.

Examples of the conductive material include carbonaceous materials suchas natural graphite, artificial graphite, cokes, carbon black, pyrolyticcarbons, carbon fiber, and a fired product of an organic polymercompound. The present embodiment may use (i) only one kind of the aboveconductive materials or (ii) two or more kinds of the above conductivematerials in combination, for example a mixture of artificial graphiteand carbon black.

Examples of the binding agent include thermoplastic resins such aspolyvinylidene fluoride, a copolymer of vinylidene fluoride,polytetrafluoroethylene, a tetrafluoroethylene-hexafluoropropylenecopolymer, a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer,an ethylene-tetrafluoroethylene copolymer, a vinylidenefluoride-hexafluoropropylene-tetrafluoroethylene copolymer, and athermoplastic polyimide, polyethylene, and polypropylene. The bindingagent functions also as a thickening agent.

The cathode mix may be prepared by, for example, a method of applyingpressure to the cathode active material, the conductive material, andthe binding agent on the cathode current collector or a method of usingan appropriate organic solvent so that the cathode active material, theconductive material, and the binding agent are in a paste form.

The cathode current collector is, for example, an electric conductorsuch as Al, Ni, and stainless steel, among which Al is preferablebecause Al is easily processed into a thin film and is inexpensive.

The sheet-shaped cathode may be produced, that is, the cathode mix maybe supported by the cathode current collector, through, for example, amethod of applying pressure to the cathode active material, theconductive material, and the binding agent on the cathode currentcollector to form a cathode mix thereon or a method of (i) using anappropriate organic solvent so that the cathode active material, theconductive material, and the binding agent are in a paste form toprovide a cathode mix, (ii) applying the cathode mix to the cathodecurrent collector, (iii) drying the applied cathode mix to prepare asheet-shaped cathode mix, and (iv) applying pressure to the sheet-shapedcathode mix so that the sheet-shaped cathode mix is firmly fixed to thecathode current collector.

The anode is normally a sheet-shaped anode including (i) an anode mixcontaining an anode active material and (ii) an anode current collectorsupporting the anode mix thereon.

The anode active material is, for example, (i) a material capable ofbeing doped and dedoped with lithium ions, (ii) a lithium metal, or(iii) a lithium alloy. Specific examples of the material includecarbonaceous materials such as natural graphite, artificial graphite,cokes, carbon black, pyrolytic carbons, carbon fiber, and a firedproduct of an organic polymer compound; and chalcogen compounds such asan oxide and a sulfide that are doped and dedoped with lithium ions atan electric potential lower than that for the cathode. Among the aboveanode active materials, a carbonaceous material containing a graphitematerial such as natural graphite or artificial graphite as a maincomponent is preferable because such a carbonaceous material has highelectric potential flatness and low average discharge potential and canthus be combined with a cathode to achieve high energy density.

The anode mix may be prepared by, for example, a method of applyingpressure to the anode active material on the anode current collector ora method of using an appropriate organic solvent so that the anodeactive material is in a paste form.

The anode current collector is, for example, Cu, Ni, or stainless steel,among which Cu is preferable because Cu is not easily alloyed withlithium in the case of a lithium ion secondary battery and is easilyprocessed into a thin film.

The sheet-shaped anode may be produced, that is, the anode mix may besupported by the anode current collector, through, for example, a methodof applying pressure to the anode active material on the anode currentcollector to form an anode mix thereon or a method of (i) using anappropriate organic solvent so that the anode active material is in apaste form to provide an anode mix, (ii) applying the anode mix to theanode current collector, (iii) drying the applied anode mix to prepare asheet-shaped anode mix, and (iv) applying pressure to the sheet-shapedanode mix so that the sheet-shaped, anode mix is firmly fixed to theanode current collector.

The nonaqueous electrolyte secondary battery of the present inventionmay be produced by (i) arranging the cathode, the laminated body, andthe anode in that order to form a nonaqueous electrolyte secondarybattery member, (ii) inserting the nonaqueous electrolyte secondarybattery member into a container for use as a housing of the nonaqueouselectrolyte secondary battery, (iii) filling the container with anonaqueous electrolyte solution, and (iv) hermetically sealing thecontainer under reduced pressure. The nonaqueous electrolyte secondarybattery may have any shape such as the shape of a thin plate (paper), adisk, a cylinder, or a prism such as a cuboid. The nonaqueouselectrolyte secondary battery may be produced through any method, andmay be produced through a conventionally publicly known method.

The nonaqueous electrolyte secondary battery of the present inventionhas a high dielectric strength, and has a low rate of defects such as acurrent leak.

The present invention is not limited to the description of theembodiments above, but may be altered in various ways by a skilledperson within the scope of the claims. Any embodiment based on a propercombination of technical means disclosed in different embodiments isalso encompassed in the technical scope of the present invention.Further, combining different technical means disclosed in differentembodiments can provide a new technical feature.

The present invention may also include in its scope a laminated bodyarranged as below, a nonaqueous electrolyte secondary battery memberarranged as below, and a nonaqueous electrolyte secondary batteryarranged as below.

[1] A laminated body, including:

a porous film containing a polyolefin as a main component; and

a porous layer on one or both surfaces of the porous film, the porouslayer containing a resin,

the laminated body satisfying

(A)>(B),

where

(A) represents an amount (V·m²/g) of an increase in a dielectricstrength of the porous layer with respect to an amount of an increase inan amount (g/m²) of the resin contained per unit area of the porouslayer, and

(B) represents an amount (V·m²/g) of an increase in a dielectricstrength of the porous film with respect to an amount of an increase inan amount (g/m²) of the polyolefin contained per unit area of the porousfilm.

[2] The laminated body according to [1],

wherein

the resin is a polyvinylidene fluoride.

[3] The laminated body according to [1],

wherein

the resin is an aromatic polyamide,

[4] A member for a nonaqueous electrolyte secondary battery,

the member including in sequence:

a cathode;

the laminated body according to any one of [1] to [3]; and

an anode.

[5] A nonaqueous electrolyte secondary battery, including

the laminated body according to any one of [1] to [3] as a separator.

Third Embodiment: Aspect 3 of Present Invention

The description below deals with a third embodiment of the presentinvention. The present invention is, however, not limited to such anembodiment. Further, the present invention is not limited to thedescription of the arrangements below, but may be altered in variousways by a skilled person within the scope of the claims. Any embodimentor example based on a proper combination of technical means disclosed indifferent embodiments and examples is also encompassed in the technicalscope of the present invention. All academic and patent documents citedin the present specification are incorporated herein by reference. Inthe present specification, any numerical range expressed as “A to B”means “not less than A and not greater than B” unless otherwise stated.

[1. Laminated Body]

A laminated body of the present invention includes a stack of (i) afirst porous layer made of a polyolefin-based resin and (ii) a secondporous layer

For the present invention, a moisture absorption property of thelaminated body is important. The inventors of the present inventiondiscovered that (i) the moisture absorption property of the laminatedbody, which is included in the separator, is closely relevant tooccurrence of curling of the separator and (ii) a laminated body havinga moisture absorption within a certain range can prevent occurrence ofthe curling. Specifically, with regard to the laminated body including astack of the first porous layer and the second porous layer, adifference between (A) a water content rate of the laminated body in anatmosphere having a dew point of 20° C. and (B) a water content rate ofthe laminated body in an atmosphere having a dew point of −30° C. ispreferably 1000 ppm or less. Further, the difference between the watercontent rates (A) and (B) is preferably 800 ppm or less, more preferably600 ppm or less, further preferably 500 ppm or less, and particularlypreferably 400 ppm or less. Values of the water content rates (A) and(B) are calculated according to the method in the later-describedExamples.

In a case where a difference between water content rates (A) and (B) ofa laminated body is the above-described value or less, i.e., in a casewhere a difference between water content rates of a laminated bodyobserved at two different dew points (20° C. and −30° C.) is apredetermined value or less, it is possible to prevent occurrence ofcurling of the laminated body which is used as the separator. Theinventors of the present invention assume that, for a mechanism of thiseffect, it is important to control the amount of change in water contentof the laminated body to be the predetermined value or less, becausecurling of the laminated body occurs if the water content of thelaminated body changes in an amount greater than the predetermined valuewhen the dew point becomes low.

Further, the difference between the water content rates (A) and (B) ofthe laminated body is preferably 100 ppm or more. In a case where adifference between water content rates (A) and (B) of a laminated bodyis 100 ppm or more, the electrolyte solution is easily permeated intothe laminated body which is used as the separator. Thus, an adverseinfluence is hardly given to rate characteristics and/or the like.

Further, as another important moisture absorption property, a differencebetween (C) a water content of the first porous layer in an atmospherehaving a dew point of 20° C. and (D) a water content of the secondporous layer in the atmosphere is preferably 10 mg/m² or less.Furthermore, the difference between the water contents (C) and (D) ispreferably 8 mg/m² or less, more preferably 7 mg/m² or less, furtherpreferably 6 mg/m² or less, and particularly preferably 5 mg/m² or less.Values of the water contents (C) and (D) are calculated according to themethod in the later-described Examples.

In a case where a difference between water contents (C) and (D) in alaminated body is the above-described value or less, it is possible toprevent occurrence of curling of the laminated body which is used as theseparator. With regard to this, the inventors of the present inventionassume that it is important to control the difference between the watercontents of the first porous layer and the second porous layer, becausecurling of the laminated body occurs if the difference between the watercontents of the first porous layer and the second porous layer, eachincluded in the laminated body, is greater than the predetermined value.Here, the less water content the second porous layer has, the morepreferable it is. This is because that, the less water content thesecond porous layer has, the less likely a side reaction occurs in thebattery due to water and therefore the less likely the batterycharacteristics are degraded.

Further, the difference between the water contents (C) and (D) ispreferably 1 mg/m² or more. In a case where a difference between watercontents (C) and (D) in a laminated body is 1 mg/m² or more, theelectrolyte solution is easily permeated into the laminated body whichis used as the separator. Thus, an adverse influence is hardly given torate characteristics and/or the like.

Further, for the present invention, a shape of the second porous layer,which is included in the laminated body, is important. The inventors ofthe present invention discovered that (i) the shape of the second porouslayer of the laminated body, which constitutes the separator, is closelyrelevant to occurrence of curling of the separator and (ii) a secondporous layer having a certain shape can prevent occurrence of thecurling. Specifically, an area of opening sections, each of which is amacropore of 0.5 μm² or more, accounts for preferably 30% or less of asurface of the second porous layer (a surface of the second porous layerbeing not a surface onto which the first porous layer is stacked), morepreferably 20% or less, further preferably 10% or less, and particularlypreferably 5% or less. Further, in terms of ion permeability, such anarea of the opening sections is preferably 0.001 or more, and morepreferably 0.1% or more. The area of the opening sections, each of whichis the macropore of 0.5 μm² or more, in the surface of the second porouslayer is calculated according to a method explained in thelater-described Examples. The area of the opening sections, each ofwhich is the macropore of 0.5 μm² or more, in the surface of the secondporous layer is preferably within the above-described range, because itis possible to effectively prevent occurrence of curling of a laminatedbody having such an arrangement.

Next, the following explains the first porous layer and the secondporous layer, which are included in the laminated body of the presentinvention.

[1-1. First Porous Layer]

The first porous layer only needs to be made of a polyolefin-basedresin, and is preferably a microporous film. Namely, the first porouslayer is preferably a porous film that (i) contains a polyolefin as amain component, (ii) has inside itself pores connected to one another,and (iii) allows a gas, a liquid, or the like to pass therethrough fromone surface to the other. Also, the first porous layer can be arrangedsuch that, in a case where the battery generates heat, the first porouslayer is melted so as to make the laminated body (i.e., the separator)non-porous. Thus, the first porous layer can impart to the laminatedbody a shutdown function. The first porous layer can be made of a singlelayer or a plurality of layers.

It is essential that the first porous layer contains a polyolefincomponent at a proportion of 50% or more by volume with respect to wholecomponents contained in the first porous layer. Such a proportion of thepolyolefin component is preferably 90% or more by volume, and morepreferably 95% or more by volume. The first porous layer preferablycontains, as the polyolefin component, a high molecular weight componenthaving a weight-average molecular weight of 5×10⁵ to 15×10⁶. The firstporous layer particularly preferably contains, as the polyolefincomponent, a polyolefin component having a weight-average molecularweight of 1,000,000 or more. This is because that (i) a first porouslayer containing such a polyolefin component and (ii) the whole of alaminated body including such a first porous layer achieve higherstrength.

Examples of the polyolefin include high molecular weight homopolymers orcopolymers produced through polymerization of ethylene, propylene,1-butene, 4-methyl-1-pentene, 1-hexene, and/or the like. The firstporous layer can be a layer containing only one of these polyolefinsand/or a layer containing two or more of these polyolefins. Among these,a high molecular weight polyethylene containing ethylene as a maincomponent is particularly preferable. Note that the first porous layercan contain other component which is not a polyolefin, as long as theother component does not impair the function of the first porous layer.

The first porous layer has inside itself pores connected to one anotherand allows a gas, a liquid, an ion or the like to pass therethrough fromone surface to the other. A transmittance thereof is normally indicatedas an air permeability. The first porous layer has an air permeabilitynormally in a range of 30 to 1000 sec/100 cc, and preferably in a rangeof 50 to 800 sec/100 cc, in terms of Gurley values. A first porous layerhaving an air permeability within such a range achieves sufficient ionpermeability in a case where the first porous layer is used in theseparator.

The first porous layer has a porosity of preferably 20 to 80% by volume,and more preferably 30 to 70% by volume, in order to allow the separatorto (i) retain a larger amount of electrolyte solution and (ii) achievethe shutdown function reliably. A first porous layer having a porosityof less than 20% by volume may decrease an amount of electrolytesolution retained by the separator. Meanwhile, a first porous layerhaving a porosity of more than 80% by volume may lead to an insufficienteffect of making the laminated body non-porous, even at a hightemperature at which shutdown should be performed. Namely, such a firstporous layer may make it impossible to prevent a current even in a casewhere the battery generates a high heat.

The first porous layer has a pore size of preferably 3 μm or less, andmore preferably 1 μm or less. This is because that, in a case where theseparator of the present invention including such a first porous layeris included in the battery, it is possible to achieve sufficient ionpermeability and to prevent particles from entering the cathode, theanode, or the like.

A thickness of the first porous layer is selected as appropriate in viewof the number of layers in the laminated body. Particularly in a casewhere the first porous layer is used as a base material and the secondporous layer is formed on one surface (or both surfaces) of the firstporous layer, the first porous layer has a thickness of preferably 4 to40 μm, and more preferably a thickness of 5 to 30 μm. A first porouslayer having a thickness of less than 4 μm may have an insufficientstrength. Meanwhile, a first porous layer having a thickness more than40 μm may lead to a small battery capacity due to its too largethickness.

The first porous layer has a weight per unit area of normally 4 to 15g/m², in and preferably 5 to 12 g/m². This is because that a firstporous layer having such a weight is possible to provide suitablestrength, thickness, handling easiness, and weight of the laminated bodyand is also possible to enhance a weight energy density and/or a volumeenergy density in a case where the first porous layer is used in theseparator of the battery.

Suitable examples of such a first porous layer include a porouspolyolefin layer disclosed in Japanese Patent Application Publication,Tokukai, No. 2013-14017 A, a polyolefin porous film disclosed inJapanese Patent Application Publication, Tokukai, No. 2012-54229 A, anda polyolefin base material porous film disclosed in Japanese PatentApplication Publication, Tokukai, No. 2014-040580 A.

The first porous layer may be produced through any publicly-knowntechnique, and is not particularly limited to any specific method. Forexample, as disclosed in Japanese Patent Application Publication,Tokukaihei, No. 7-29563 A (1995), the first porous layer may be producedthrough a method of (i) adding a plasticizing agent to a thermoplasticresin to shape the thermoplastic resin into a film and then (ii)removing the plasticizing agent with use of an appropriate solvent.

Specifically, in a case of, for example, producing a first porous layerwith use of a polyolefin resin containing (i) an ultra high molecularweight polyethylene and (ii) a low molecular weight polyolefin having aweight-average molecular weight of 10,000 or less, such a first porouslayer is, in terms of production cost, preferably produced through themethod including the steps of:

(a) kneading (i) 100 parts by weight of the ultra high molecular weightpolyethylene, (ii) 5 to 200 parts by weight of the low molecular weightpolyolefin having a weight-average molecular weight of 10,000 or less,and (iii) 100 to 400 parts by weight of an inorganic filler of calciumcarbonate or the like to produce a polyolefin resin composition,

(b) shaping the polyolefin resin composition into a sheet,

(c) removing the inorganic filler from the sheet produced in the step(b), and

(d) drawing the sheet produced in the step (c) to produce a first porouslayer.

Alternatively, the first porous layer may be produced through any of themethods explained in the above-described Patent Literatures.

The first porous layer may alternatively be a commercially availableproduct having the above physical properties.

Adjustment of the water content rate of the first porous layer can bemade by selecting a raw material of the first porous layer.Alternatively, such adjustment can be made by performing ahydrophilization treatment on the first porous layer. Performing thehydrophilization treatment so that the first porous layer attains adesired water content rate to yield the laminated body of the presentinvention. Examples of the hydrophilization treatment include (i) achemical treatment involving an acid, an alkali, or the like, (ii) acorona treatment, and (iii) a plasma treatment.

[1-2. Second Porous Layer]

The second porous layer only needs to exhibit the above-describedmoisture absorption property, and is not particularly limited in termsof its specific arrangement. The second porous layer can be a layer (i)having inside itself pores connected to one another and (ii) allowing agas, a liquid, or the like to pass therethrough from one surface to theother. Further, the second porous layer may be a layer which (i) isprovided on one surface of the first porous layer as an outermost layerof the laminated body and (ii) can be adhered to the electrode in a casewhere the second porous layer is used in the separator.

The second porous layer may include a plurality of layers. For example,the second porous layer can include at least one of a heat-resistantlayer and a functional layer. In a case of a second porous layerincluding the heat-resistant layer and the functional layer, theheat-resistant layer may be disposed between the first porous layer andthe functional layer (namely, a laminated body including the firstporous layer, the heat-resistant layer, and the functional layer whichare stacked in this order is obtained). A laminated body according toanother aspect of the present invention may include the first porouslayer, the functional layer, and the heat-resistant layer which arestacked in this order. A laminated body according to further anotheraspect of the present invention may include the functional layer and theheat-resistant layer by which the first porous layer is sandwiched(namely, the heat-resistant layer, the first porous layer, and thefunctional layer are stacked in this order). However, in order to attainthe separator having a small thickness, the second porous layerpreferably includes the functional layer only. Such an arrangementcontributes to a higher capacity of the battery.

The following description deals with the functional layer and theheat-resistant layer.

Adjustment of the water content rate of the second porous layer can bemade by selecting a raw material of the second porous layer.Alternatively, such adjustment can be made by performing ahydrophilization treatment on the second porous layer. Performing thehydrophilization treatment so that the second porous layer attains adesired water content rate to yield the laminated body of the presentinvention. Examples of the hydrophilization treatment include (i) achemical treatment involving an acid, an alkali, or the like, (ii) acorona treatment, and (iii) a plasma treatment.

<Functional Layer>

The functional layer included in the second porous layer only needs toexhibit the above-described moisture absorption property, and is notparticularly limited in terms of its specific arrangement. For example,the functional layer is preferably made of a resin having a structure inwhich skeletons each having a diameter of 1 μm or less are bonded toeach other in a three-dimensional network. For example, such a resin ispreferably the one containing a polyvinylidene fluoride-based resin(hereinafter, such a resin may also be simply referred to as “PVDF-basedresin”).

Examples of the PVDF-based resin include homopolymers of vinylidenefluoride (i.e., polyvinylidene fluoride); copolymers (e.g.,polyvinylidene fluoride copolymer) of vinylidene fluoride and othermonomer(s) polymerizable with vinylidene fluoride; and mixtures of thesepolymers. Examples of the monomer polymerizable with vinylidene fluorideinclude hexafluoropropylene, tetrafluoroethylene, trifluoroethylene,trichloroethylene, and vinyl fluoride. The present invention, can use(i) one kind of monomer or (ii) two or more kinds of monomers selectedfrom above. The PVDF-based resin can be synthesized through emulsionpolymerization or suspension polymerization.

The PVDF-based resin preferably contains vinylidene fluoride at aproportion of 95 mol % or more (more preferably, 98 mol % or more). APVDF-based resin containing vinylidene fluoride at a proportion of 95mol % or more is more likely to allow the second porous layer to achievea mechanical strength and a heat resistance against a pressure or heatoccurred in battery production.

A functional layer according to another preferable aspect contains twokinds of PVDF-based resins (a first resin and a second resin below) thatare different from each other in a content of hexafluoropropylene.

The first resin is (i) a vinylidene fluoride-hexafluoropropylenecopolymer containing hexafluoropropylene at a proportion of more than 0mol % and 1.5 mol % or less or (ii) a vinylidene fluoride homopolymer(containing hexafluoropropylene at a proportion of 0 mol %).

The second resin, is a vinylidene fluoride-hexafluoropropylene copolymercontaining hexafluoropropylene at a proportion of more than 1.5 mol %.

The functional layer containing the two kinds of PVDF-based resins isadhered to the electrode more favorably, as compared with a functionallayer not containing one of the two kinds of PVDF-based resins. Further,the functional layer containing the two kinds of PVDF-based resins hasimproved adhesiveness to the first porous layer and is separated fromthe first porous layer more favorably, as compared with a functionallayer not containing one of the two kinds of PVDF-based resins. Thefirst resin and the second resin are preferably mixed at a mixing ratio(mass ratio, first resin: second resin) of 15:85 to 85:15.

The PVDF-based resin has a weight-average molecular weight of preferably300,000 to 3,000,000. A PVDF-based resin having a weight-averagemolecular weight of 300,000 or more allows the second porous layer toattain a mechanical property with which the second porous layer canendure a process for adhering the second porous layer to the electrode,thereby allowing the second porous layer and the electrode to adhere toeach other sufficiently. Meanwhile, a PVDF-based resin having aweight-average molecular weight of 3,000,000 or less does not cause thecoating solution, which is to be applied in order to shape the secondporous layer, to have a too high viscosity, which allows the coatingsolution to have excellent shaping easiness. The weight-averagemolecular weight of the PVDF-based resin is more preferably 300,000 to2,000,000, and further preferably 500,000 to 1,500,00,000.

The PVDF-based resin has a fibril diameter of preferably 10 nm to 1000nm, in terms of the cycle characteristic.

The functional layer may contain other resin which is not the PVDF-basedresin. Examples of the other resin include styrene-butadiene copolymer;homopolymers or copolymers of vinyl nitriles such as acrylonitrile andmethacrylonitrile; and polyethers such as polyethylene oxide andpolypropylene oxide.

Further, the functional layer may contain a filler made of inorganicmatter or organic matter. A functional layer containing the filler canimprove slidability and/or heat resistance of the separator. The fillermay be an organic filler or an inorganic filler each of which is stablein a nonaqueous electrolyte solution and is electrochemically stable.

Examples of the organic filler include crosslinked high molecule fineparticles such as crosslinked polyacrylic acid, crosslinked polyacrylicacid ester, crosslinked polymethacrylic acid, crosslinkedpolymethacrylic acid ester, crosslinked polymethyl methacrylate,crosslinked polysilicone, crosslinked polystyrene, crosslinkedpolydivinyl benzene, a crosslinked product of a styrene-divinylbenzenecopolymer, polyimide, a melamine resin, a phenol resin, abenzoguanamine-formaldehyde condensate; and heat-resistant high moleculefine particles such as polysulfone, polyacrylonitrile, polyaramid,polyacetal, and thermoplastic polyimide.

A resin (high molecule) contained in the organic filler may be amixture, a modified product, a derivative, a copolymer (a randomcopolymer, an alternating copolymer, a block copolymer, or a graftcopolymer), or a crosslinked product of any of the molecules listedabove as examples.

Examples of the inorganic filler include metal hydroxides such asaluminum hydroxide, magnesium hydroxide, calcium hydroxide, chromiumhydroxide, zirconium hydroxide, nickel hydroxide, and boron hydroxide;metal oxides such as alumina and zirconia; carbonates such as calciumcarbonate and magnesium carbonate; sulfates such as barium sulfate andcalcium sulfate; clay minerals such as calcium silicate and talc. Amongthese, the inorganic filler is preferably a metal hydroxide, in terms ofachievement of fire retardance and/or electricity removal effects.

The present invention may use (i) only one of filler or (ii) two or moreof fillers in combination.

The filler has a volume average particle size of preferably 0.01 μm to10 μm, in order to ensure (i) fine adhesion and fine slidability and(ii) shaping easiness of the separator. A lower limit of the volumeaverage particle size is more preferably 0.1 μm or more, whereas anupper limit of the volume average particle size is more preferably 5 μmor less.

The filler is constituted by particles of any shape, which may be asphere, an ellipse, a plate-shape, a bar-shape, or an irregular shape.In order to prevent a short circuit in the battery, the particles arepreferably (i) bar-shaped particles or (ii) primary particles which arenot aggregated.

The filler forms fine bumps on a surface of the functional layer,thereby improving the slidability. A filler constituted by (i)plate-shaped particles or (ii) primary particles which are notaggregated forms finer bumps on the surface of the functional layer, sothat the functional layer is adhered to the electrode more favorably.

The functional layer contains the filler at a proportion of preferably1% by mass to 30% by mass with respect to a total amount of thePVDF-based resin and the filler. A functional layer containing thefiller at a proportion of 1% or more by mass is likely to exhibit theeffect of forming fine bumps on the surface of the functional layer soas to improve the slidability of the separator. From this viewpoint, thefunctional layer contains the filler more preferably at a proportion of3% or more by mass. Meanwhile, a functional layer containing the fillerat a proportion of 30% or less by mass allows each of the functionallayer and the separator to maintain mechanical strength. With thisarrangement, for example, during a process for producing an electrodebody by rolling up a stack of the electrode and the separator, theseparator is hardly cracked and/or the like. From this viewpoint, thefunctional layer contains the filler at a proportion of more preferably20% or less by mass, and further preferably 10% or less by mass.

In order to prevent, in a process of slitting the separator, a slittedsurface of the separator from becoming fibrous, bending, and/orpermitting intrusion of scraps occurred as a result of the slitting, thefunctional layer contains the filler at a proportion of preferably 1% ormore by mass, and more preferably 3% or more by mass, with respect to atotal amount of the PVDF-based resin and the filler.

In order to ensure adhesion to the electrode and a high energy density,the functional layer has, on one surface of the first porous layer, anaverage thickness of preferably 0.5 μm to 10 μm, and more preferably 1μm to 5 μm.

The functional layer is preferably made porous sufficiently, in terms ofion permeability. Specifically, the functional layer has a porosity ofpreferably 30% to 60%. The functional layer has an average pore size of20 nm to 100 nm.

The functional layer has a surface roughness, in terms of a ten-pointaverage roughness (Rz), of preferably 0.8 μm to 8.0 μm, more preferably0.9 μm to 6.0 μm, and further preferably 1.0 μm to 3.0 μm. The ten-pointaverage roughness (Rz) is a value measured by a method according to JISB 0601-1994 (or Rzjis of JIS B 0601-2001). Specifically, “Rz” is a valuemeasured by ET4000 (available from Kosaka Laboratory Ltd.) with ameasurement length of 1.25 mm, a measurement rate of 0.1 mm/sec, and atemperature and humidity of 25° C./50% RH.

The functional layer has a coefficient of kinetic friction of preferably0.1 to 0.6, more preferably 0.1 to 0.4, and further preferably 0.1 to0.3. The coefficient of kinetic friction is a value measured by a methodaccording to JIS K7125. Specifically, a coefficient of kinetic frictionin the present invention is a value measured by Surface Property Tester(available from Heidon).

An applied amount of the functional layer is, on one surface of thefirst porous layer, preferably 0.5 g/m² to 1.5 g/m² in terms of adhesionto the electrode and ion permeability.

Further, the functional layer, which is included in the second porouslayer, may contain fine resin particles. The fine resin particles arepreferably made of a resin or a PVDF-based resin each having a structureunit derived from α-olefin having 2 to 4 carbon atoms. Further, such afunctional layer may contain a binder resin in addition to the fineresin particles. The binder resin is preferably a polymer (i.e., abinder resin) that (i) has a nature of causing the fine resin, particlesto be bound to each other, (ii) is insoluble in the electrolyte solutionof the battery, and (iii) is electrochemically stable while the batteryis in use. The binder resin may be a water-soluble polymer or awater-insoluble polymer.

Examples of the binder resin include: polyolefins such as polyethyleneand polypropylene; fluorine-containing resins such as polyvinylidenefluoride and polytetrafluoroethylene; fluorine-containing rubbers suchas vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymerand ethylene-tetrafluoroethylene copolymer); rubbers such asstyrene-butadiene copolymer and a hydride thereof, methacrylic acidester copolymer, acrylonitrile-acrylic acid ester copolymer,styrene-acrylic acid ester copolymer, ethylene propylene rubber, andpolyvinyl acetate; resins with a melting point or a glass transitiontemperature of 180° C. or higher, such as polyphenylene ether,polysulfone, polyether sulfone, polyphenylene sulfide, polyetherimide,polyamide, polyimide, polyamide imide, polyetheramide, and polyester;and polymers such as polyvinyl alcohol, polyethyleneglycol, celluloseethers, sodium alginate, polyacrylic acid, polyacrylamide, andpolymethacrylic acid. The present invention may use (i) only one kind ofbinder resin or (ii) a mixture of two or more of binder resins selectedfrom the above. Examples of the cellulose ether include carboxymethylcellulose (CMC), hydroxyethyl cellulose (HEC), carboxy ethyl cellulose,methyl cellulose, ethyl cellulose, cyan ethyl cellulose, and oxyethylcellulose.

A functional layer according to another aspect may be a fine particleaggregate layer containing a resin or a PVDF-based resin each having astructure unit derived from α-olefin having 2 to 4 carbon atoms. Such afunctional layer further contains a filler containing at least one of anorganic compound and an inorganic compound. In the first embodiment, acontent of the filler in the functional layer is 20% by mass to 80% bymass with respect to a total mass of the fine particles and the filler.Meanwhile, in the second embodiment, a content of the filler in thefunctional layer is 15%, by volume to 90% by volume with respect to atotal volume of the fine particles and the filler. Further, a content ofthe fine particles in a single functional layer is 0.1 g/m² to 6.0 g/m².The filler may be suitably selected from the above-exemplified ones.

The functional layer arranged above allows the separator to excel in ionpermeability and handling easiness, and makes it possible to attainfavorable adhesion between the electrode and the separator after theelectrode and the separator are bonded to each other through hotpressing.

The functional layer may be a fine particle aggregate layer containingan aggregate of the fine particles. Here, examples of the “fine particleaggregate layer” include the following arrangements (I) and (II).

In the arrangement (I), (a) the fine particles are fixed onto the firstporous layer by the primary particles or (b) the fine particles and/orthe filler are fixed onto the first porous layer as an aggregate(secondary particles).

In the arrangement (ii), a layer made of (a) a plurality of adjacentfine particles being integrally bonded to each other or (b) the fineparticles and the filler being integrally bonded to each other is fixedonto a surface of the first porous layer via at least part of the fineparticles in the layer, so that the whole of the layer is fixed onto(integrated into) the first porous layer.

The fine particles constituting the aggregate may be confirmed byobservation of a surface of the separator (a surface of the functionallayer) with a scanning electron microscope (SEM).

A structure of the functional layer is not particularly limited, as longas the functional layer has sufficient ion permeability. In terms of theion permeability, the functional layer is preferably made porous. Thefunctional layer which has been made porous is also referred as anadhesive porous layer.

The fine particles preferably retain a particle shape in the functionallayer.

The expression “retaining a particulate shape” refers to, for example, astate where particle interfaces of the fine particles are identifiablein a case where the nonaqueous secondary battery separator of thepresent invention is observed with the scanning electron microscope.

The fine particles have an average particle size of preferably 0.01 μmto 1 μm, more preferably 0.02 μm to 1 μm, and particularly preferably0.05 μm to 1 μm.

Fine particles having an average particle size of 0.01 μm or more allowthe nonaqueous secondary battery separator to excel in slidability andhandling easiness. Meanwhile, fine particles having an average particlesize of 1 μm or less make it easier to provide a functional layer havinga uniformly small thickness.

A mass of the fine particles in a single functional layer is 0.1 g/m² to6.0 g/m². Preferably, the mass of the fine particles in a singlefunctional layer is in a range from 1.0 g/m² to 3.0 g/m².

In a case where the mass of the fine particles in a single functionallayer is 0.1 g/m² or more, adhesion between the separator and theelectrode is enhanced. Meanwhile, in a case where the mass of the fineparticles in a single functional layer is 6.0 g/m² or less, theseparator allows an ion to pass therethrough more easily and the loadcharacteristic of the battery is improved.

The functional layer is bonded to the electrode through pressure bondingor hot pressing per formed in a state where the functional layercontains the electrolyte solution.

The fine resin particles are preferably made of a resin having astructure unit derived from α-olefin having 2 to 4 carbon atoms. Forexample, the fine resin particles are preferably made of a copolymer ofethylene and vinyl acetate.

The fine particles has an average particle size of 0.01 μm to 1 μm, morepreferably 0.02 μm to 1 μm, and particularly preferably 0.05 μm to 1 μm.

Examples of α-olefin having 2 to 4 carbon atoms include ethylene,propylene, and 1-butene. Such α-olefin is preferably ethylene. The resinhaving a structure unit derived from α-olefin having 2 to 4 carbon atomsmay be a copolymer of (i) any of the α-olefins having 2 to 4 carbonatoms and (ii) other monomer. Examples of the other monomer includefatty acid vinyl such as vinyl acetate, propionate vinyl, butyric acidvinyl, lauric acid vinyl, caproic acid vinyl, stearic acid vinyl,palmitic acid vinyl, and versatic acid vinyl; acrylic acid ester havingan alkyl group having 1 to 16 carbon atoms such as methyl acrylate,ethyl acrylate, propyl acrylate, butyl acrylate, hexyl acrylate, octylacrylate, and lauryl acrylate; methacrylic acid ester having an alkylgroup having 1 to 16 carbon atoms such as ethyl methacrylate, propylmethacrylate, buty methacrylate, hexyl methacrylate, octyl methacrylate,and lauryl methacrylate; an acidic group-containing vinyl monomer suchas acrylic acid, methacrylic acid, 2-acryloyloxyethyl succinate,2-methacryloyloxyethyl succinate, carboxy ethyl acrylate, and carboxyethyl methacrylate; an aromatic vinyl monomer such as styrene, benzylacrylate, and benzyl methacrylate; diene such as 1,3-butadiene andisoprene; and acrylonitrile. Among these, the other monomer ispreferably fatty acid vinyl, acrylic acid ester, or methacrylic acidester, and more preferably vinyl acetate or ethyl acrylate.

The resin having a structure unit derived from α-olefin having 2 to 4carbon atoms is preferably a resin having (i) a structure unit derivedfrom α-olefin having 2 to 4 carbon atoms and (ii) a structure unitderived from the other monomer. Such a resin is more preferably a resinhaving (i) a structure unit derived from at least one selected from agroup consisting of fatty acid vinyl, acrylic acid ester, andmethacrylic acid ester and (ii) a structure unit derived from α-olefinhaving 2 to 4 carbon atoms.

<Heat-Resistant Layer>

The heat-resistant layer is not particularly limited in configurationexcept that the heat-resistant layer only needs to contain a heatresistance material so as to be heat resistant at high temperatures atwhich a shutdown occurs. Preferably, the heat-resistant layer is also alayer (i) having inside itself pores connected to one another and (ii)allowing a gas, a liquid, or the like to pass therethrough from onesurface to the other.

In a case where the second porous layer includes the heat-resistantlayer, the second porous layer can have shape stability even at hightemperatures. Note that in the present specification, a heat resistancematerial is defined as a material that does not melt or pyrolize attemperatures at which the first porous layer melts (for example,approximately 130° C. in a case where the first porous layer is made ofpolyethylene).

Examples of the heat resistance material include a heat-resistant resinand a heat-resistant resin composition which includes a filler.

Examples of the heat-resistant resin include polyamide, polyimide,polyamide imide, polycarbonate, polyacetal, polysulfone, polyphenylenesulfide, polyether ether ketone, aromatic polyester, polyether sulfone,polyetherimide, cellulose ethers. The present invention may use (i) onlyone kind of heat-resistant resin or (ii) mixture of two or more kinds ofheat-resistant resins selected from the above.

Among the above heat-resistant resins, in order to further increase heatresistance, (i) polyamide, polyimide, polyamide imide, polyethersulfone, and polyetherimide are preferable, (ii) polyamide, polyimide,and polyamide imide are more preferable, (iii) nitrogen-containingaromatic polymers such as aromatic polyamide (para-oriented aromaticpolyamide, meta-oriented aromatic polyamide), aromatic polyimide, andaromatic polyamide imide are even more preferable, and (iv) aromaticpolyamides are further preferable. From the viewpoint of heatresistance, para-oriented aromatic polyamide (hereinafter referred toalso as “para-aramid”) is particularly preferable.

Para-aramid is obtained by condensation polymerization of para-orientedaromatic diamine and para-oriented aromatic dicarboxylic acid halide,and substantially includes repeating units in which amide bonds arebonded at para positions or corresponding oriented positions (forexample, oriented positions that extend coaxially or parallel inopposite directions such as the cases of 4,4′-biphenylene,1,5-naphthalene, and 2,6-naphthalene) of aromatic rings. Examples of thepara-aramid include para-aramids each having a para-oriented structureor a structure corresponding to a para-oriented structure, such aspoly(paraphenylene terephthalamide), poly(parabenzamide),poly(4,4′-benzanilide terephthalamide),poly(paraphenylene-4,4′-biphenylene dicarboxylic acid amide),poly(paraphenylene-2,6-naphthalene dicarboxylic acid amide),poly(2-chloro-paraphenylene terephthalamide), and paraphenyleneterephthalamide/2,6-dichloroparaphenylene terephthalamide copolymer.

The aromatic polyimide is preferably fully aromatic polyimide producedthrough condensation polymerization of an aromatic diacid anhydride anda diamine. Examples of the diacid anhydride include pyromelliticdianhydride, 3,3′,4,4′-diphenyl sulfone tetracarboxylic dianhydride,3,3′,4,4′-benzophenone tetracarboxylic dianhydride,2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane, and 3,3′4,4′-biphenyltetracarboxylic dianhydride. Examples of the diamine includeoxydianiline, paraphenylenediamine, benzophenone diamine,3,3′-methylenedianiline, 3,3′-diaminobenzophenone, 3,3′-diaminodiphenylsulfone, and 1,5′-naphthalene diamine.

The aromatic polyamide imide is, for example, produced throughcondensation polymerization of (i) aromatic dicarboxylic acid andaromatic diisocyanate or (ii) aromatic diacid anhydride and aromaticdiisocyanate. Examples of the aromatic dicarboxylic acid includeisophthalic acid and terephthalic acid. Examples of the aromatic diacidanhydride include trimellitic anhydride. Examples of the aromaticdiisocyanate include 4,4′-diphenylmethane diisocyanate, 2,4-tolylenediisocyanate, 2,6-tolylene diisocyanate, ortho tolylene diisocyanate,and m-xylene diisocyanate.

Examples of the cellulose ether include carboxymethyl cellulose (CMC),hydroxyethyl cellulose (HEC), carboxy ethyl cellulose, methyl cellulose,ethyl cellulose, cyan ethyl cellulose, and oxyethyl cellulose.

Among these, CMC and HEC, which have excellent chemical and thermalstability, are preferable, and CMC is more preferable.

The filler may be an organic filler or an inorganic filler. Examples ofthe organic filler include fine particles made of: (i) a homopolymer ofa monomer such as styrene, vinyl ketone, acrylonitrile, methylmethacrylate, ethyl methacrylate, glycidyl methacrylate, glycidylacrylate, or methyl acrylate, or (ii) a copolymer of two or more of suchmonomers; fluorine-based resins such as polytetrafluoroethylene,ethylene tetrafluoride-propylene hexafluoride copolymer,tetrafluoroethylene-ethylene copolymer, and polyvinylidene fluoride(polyvinylidene fluoride-based resin); melamine resin; urea resin;polyethylene; polypropylene; or polymethacrylate.

Examples of the inorganic filler includes fine particles made of calciumcarbonate, talc, clay, kaolin, silica, hydrotalcite, diatomaceous earth,magnesium carbonate, barium carbonate, sulfuric acid calcium, magnesiumsulfate, barium sulfate, aluminum hydroxide, magnesium hydroxide,calcium oxide, magnesium oxide, titanium oxide, alumina (for example,α-alumina), mica, zeolite, or glass.

Alternatively, it is possible to use a material, such as a hydrate ofthe filler, which is similar to the filler. The present invention mayuse (i) only one kind of filler or (ii) a mixture of two or more offillers selected from the above.

Among these fillers, in order to further increase chemical stability andhigh temperature shape stability, the filler is preferably made of aninorganic oxide, and more preferably made of α-alumina.

Note that the filler is preferably a filler having a sufficient amountof pores to ensure ion permeability in the second porous layer.

Of a total 100% by weight of the heat-resistant resin and the filler, aweight proportion of the filler is 20 to 99% by weight, but ispreferably 30 to 99% by weight, more preferably 40 to 99% by weight,even more preferably 50 to 99% by weight, and still more preferably 60to 99% by weight. In a case where the weight proportion of the fillerfalls within such a certain range, it is possible to obtain a secondporous layer having an excellent balance between ion permeability andimprobability of powder falling. Note that powder falling is aphenomenon in which a filler is peeled off of a deposited porous film.

The heat-resistant layer can contain a component other than a heatresistance material, provided that the function of the heat-resistantlayer is not impaired. Examples of such a component include a dispersingagent, a plasticizing agent, and a pH adjusting agent.

A thickness of the heat-resistant layer is 1 to 25 μm or less, but ispreferably in a range of 5 to 20 μm or less. If the thickness of theheat-resistant layer is 1 μm or more, then thermal shrinkage of thefirst porous layer, which occurs when heat is generated in the batteryby an accident or the like, can be prevented. This avoids thecontraction of the separator. Meanwhile, if the thickness of theheat-resistant layer is 25 μm or less, then the second porous layer isnot excessively thick. This avoids a risk of causing a capacity of thebattery to be small.

[2. Separator Including Laminated Body]

The laminated body of the present invention can be used as a separator(for example, nonaqueous secondary battery separator). Such a separatorpreferably has an overall thickness of 5 μm to 35 μm, more preferably 10μm to 20 μm, from the viewpoint of (i) mechanical strength and (ii)energy density in a case where the separator is included in a battery.

A porosity of the separator is preferably 30% to 60% from the viewpointsof (i) adhesion to electrode, (ii) handling easiness, (iii) mechanicalstrength, and (iv) ion permeability.

A Gurley value (JIS P8117) of the separator is preferably in a range of50 seconds/100 cc to 800 seconds/100 cc since a balance betweenmechanical strength and film resistance is good in such a range. Fromthe viewpoint of ion permeability, a difference between a Gurley valueof a first porous layer and a Gurley value of the separator, in which asecond porous layer is provide on the first porous layer, is preferablynot more than 300 seconds/100 cc, more preferably not more than 150seconds/100 cc, and even more preferably not more than 100 seconds/100cc.

A tortuosity ratio of the separator is preferably 1.5 to 2.5 from theviewpoint of ion permeability.

From the viewpoint of load characteristics of the battery, the filmresistance of the separator is preferably 1 ohm·cm² to 10 ohm·cm². Thefilm resistance as defined herein is a resistance value in a case wherethe separator is impregnated with an electrolyte solution, and ismeasured through an alternating current method. While the abovenumerical value varies naturally depending on a kind of electrolytesolution and on temperature, the above numerical value defined herein isa numerical value measured at 20° C. with use of 1M LiBF₄-propylenecarbonate/ethylene carbonate (mass ratio: 1/1) as an electrolytesolution.

A thermal shrinkage rate of the separator at 105° C. in each of an MDdirection and a TD direction is preferably 10% or less. In a case wherethe thermal shrinkage rate falls within this range, shape stability anda shutdown characteristic of the separator are balanced. The thermalshrinkage rate is more preferably 5% or less.

[3. Method for Producing Separator]

A method for producing the laminated body of the present invention isnot limited to any particular one, but may be selected from variousmethods, provided that the separator can be obtained.

For example, in a case where the functional layer of the second porouslayer is made of PVDF-based resin, the separator can be produced througha method in which a second porous layer is integrally formed on a firstporous layer by (i) forming a coating layer by applying a coatingsolution, which contains a PVDF-based resin, to the first porous layer(base material) and then (ii) curing the PVDF-based resin in the coatinglayer.

A second porous layer containing a PVDF-based resin may be formedthrough, for example, the wet coating method below. First, a PVDF-basedresin is dissolved in a solvent, and as necessary, a filler is dispersedin the solution to prepare a coating solution. This coating solution isapplied to a first porous layer, and then the first porous layer withthe coating solution applied is immersed in an appropriate settingliquid to induce phase separation and cure the PVDF-based resin.Performing these steps forms, on the first porous layer, a layercontaining a PVDF-based resin and having a porous structure. This layeris then washed with water and dried to remove the setting liquid fromthe layer having a porous structure.

A specific example is the following method:

(Method 1)

(a) Prepare a solution in which a PVDF-based resin is dissolved in asolvent.(b) Apply the solution to a first porous layer to form a coating film.(c) Perform an operation such as immersing, into a solvent that does notdissolve the PVDF-based resin, the wet coating film to separate thePVDF-based resin out of the coating film.(d) As necessary, immerse the wet coating film, in which the PVDF-basedresin has been separated, again into a solvent that does not dissolvethe PVDF-based resin and wash the coating film.(e) Dry the wet coating film, in which the PVDF-based resin has beenseparated.

(Method 2)

(a) Prepare a coating film in which a PVDF-based resin is dispersed inan aqueous solution which is obtained by, as needed, dissolving a binderresin in water. In so doing, the PVDF-based resin can be fine particles.(b) Apply the solution to the first porous layer to form a coating film.(c) Dry the coating film to remove water.

A second porous layer produced through such a method normally has astructure in which skeletons each having a diameter of 1 μm or less arebonded to each other in a three-dimensional network. Whether or not thesecond porous layer has a structure in which skeletons each having adiameter of 1 μm or less are bonded to each other in a three-dimensionalnetwork can be examined by observing a surface of the second porouslayer with the use of a scanning electron microscope.

The description below deals in detail with a wet coating method suitablefor the present invention.

Suitable examples of the solvent (hereinafter referred to also as “goodsolvent”) for use in the preparation of the coating solution whichsolvent dissolves a PVDF-based resin include polar amide solvents suchas N-methyl-2-pyrrolidone (NMP), dimethylacetamide, dimethylformamide,and dimethylformamide.

To form a good porous structure, the good solvent is preferably mixedwith a phase separating agent for inducing phase separation. Examples ofthe phase separating agent include water, methanol, ethanol, propylalcohol, and butyl alcohol. Note, however, that a hydrophilic phaseseparating agent, such as tripropylene glycol and ethylene glycol, whichhas a boiling point of more than 150° C., is preferably not mixed withthe good solvent. A first porous layer made of polyolefin resin ismelted and deformed at 80° C. to 150° C. Therefore, the laminated bodyof the present invention cannot be dried at temperatures higher than150° C. This tends to cause a hydrophilic phase separating agent havinga boiling point of higher than 150° C. to remain in a laminated body, sothat a water content rate of the laminated body in an atmosphere havinga dew point of 20° C. tends to be increased. The phase separating agentis preferably added in an amount that achieves viscosity suitable forthe coating.

To form a good porous structure, the solvent is preferably a mixedsolvent containing (i) 60% or more by mass of a good solvent and (ii) 5%by mass to 40% by mass of a phase separating agent. To form a goodporous structure, the coating solution preferably contains a PVDF-basedresin at a concentration of 3% by mass to 10% by mass.

To impart slidability to the second porous layer and form a good porousstructure, the coating solution contains a filler at a proportion ofpreferably 1% by mass to 30% by mass, more preferably 3% by mass to 28%by mass, with respect to the total amount of the PVDF-based resin andthe filler.

The setting liquid (solvent that does not dissolve a PVDF-based resin)typically includes (i) a good solvent and phase separating agent for usein the preparation of the coating solution and (ii) water. It is generalpractice that the good solvent and the phase separating agent is mixedat a ratio equal to that of the mixed solvent for use in the dissolutionof the PVDF-based resin. For good formation of a porous structure andproductivity, the water concentration is preferably 40% by mass to 90%by mass. For the reason similar to that in the case of the good solvent,a hydrophilic phase separating agent having a boiling point of higherthan 150° C. is also not contained in the setting liquid.

The coating solution may be applied to the first porous layer through aconventional coating method, for example, with use of a Meyer bar, a diecoater, a reverse roll coater, or a gravure coater.

The second porous layer may alternatively be produced through a drycoating method instead of a wet coating method described above. The drycoating method is a method of (i) applying to a second porous layer acoating solution containing a PVDF-based resin, a filler, and a solventand then (ii) drying the coating layer for removal of the solventthrough volatilization to produce a porous layer. The dry coatingmethod, however, (i) likely produces a closely packed coating layer ascompared to the wet coating method and (ii) likely causes the goodsolvent in the coating solution to remain in the second porous material.Thus, to form a good porous structure, the wet coating method ispreferable.

In the dry coating method, a mixed solvent, in which a good solvent ismixed with a poor solvent having a boiling point higher than that of thegood solvent, may be used as a solvent that dissolves a PVDF-based resinto form a porous structure. From the viewpoint of prevention of a curlof a laminated body, however, such a mixed solvent is not preferable. Ina case where such a mixed solvent is used, a good solvent evaporatesfirst, and then a poor solvent remains. This causes pores to be formedin a porous layer. That is, parts of the poor solvent, on which partsthe poor solvent was present before the evaporation, become pores. In acase where a second porous layer is formed through such a method, porestend to be large in size. Specifically, an area of opening sections,each of which is a macropore of 0.5 μm² or more, tends to account formore than 30% of a surface of the second porous layer.

In a case where (i) the pores are large in size and (ii) the area ofopening sections, each of which is a macropore of 0.5 μm² or more,accounts for more than 30% of the surface of the second porous layer,pores of the second porous layer, which pores are located on aninterface between the first porous layer and the second porous layer,also become large in size. This causes an adhesion point between thefirst porous layer and the second porous layer to be rough.Consequently, contraction stress of the first porous layer, which occursas a result of a change in humidity, cannot be restricted by the secondporous layer. This causes the laminated body to curl.

The separator may alternatively be produced through a method of (i)preparing a second porous layer as a separate sheet, (ii) placing thesecond porous layer on a first porous layer, and (iii) combining thesecond porous layer with the first porous layer throughthermocompression or with use of an adhesive. The second porous layermay be prepared as a separate sheet through a method of, for example,(i) applying to a release sheet a coating solution containing aPVDF-based resin and a filler, (ii) forming a second porous layerthrough the wet coating method or dry coating method described above,and (iii) peeling the second porous layer from the release sheet.

In a case where the second porous layer is a heat-resistant layer,examples of a method to be employed include: a method of (i) preparing afirst porous layer and a second porous layer separately and (ii)combining the first porous layer and the second porous layer with eachother; and a method of (i) preparing a coating solution containing amedium as well as a component for a second porous layer, (ii) applyingthe coating solution to a first porous layer, and (iii) removing themedium. The latter of these methods is simple and preferable.

The medium is a solvent or a disperse medium, and simply needs to becapable of dissolving or dispersing a component for a second porouslayer uniformly and stably. Examples of the medium include water,alcohols such as methanol, ethanol, and isopropanol, acetone, toluene,xylene, hexane, N-methylpyrrolidone, N,N-dimethylacetamide, andN,N-dimethylformamide. The present invention may use only one of theabove mediums, or mix two or more of the above mediums with each otherfor use as long as the two or more mediums are dissolved in each other.

In terms of process or environmental load, it is preferable for themedium to contain water at 80% or more by weight, more preferablycontain only water.

The coating solution may be applied to the first porous layer throughany method that allows uniform wet coating, and may be applied through aconventionally publicly known method. Examples of the application methodinclude capillary coating method, spin coating method, slit die coatingmethod, spray coating method, roll coating method, screen printingmethod, flexographic printing method, bar coater method, gravure coatermethod, and die coater method. The thickness of the second porous layermay be controlled by adjusting (i) the amount of the coating solution tobe applied, (ii) the concentration of the polymer in the coatingsolution, and/or (iii) in a case where the coating solution containsfine particles, the ratio of the fine particles to the polymer.Normally, a process of (i) applying the coating solution to the firstporous layer and (ii) removing, from the coating solution, a mediumwhich has been applied to the first porous layer, is carried, outsequentially while the first porous layer is conveyed. This allows thefirst porous layer and the second porous layer to be sequentiallydeposited even in a case where the first porous layer is long.

The coating solution may be prepared through any method that allows ahomogeneous coating solution to be prepared. In a case where the coatingsolution is to contain a component in addition to a polymer other thanpolyolefin, particularly a filler, the coating solution is preferablyprepared through a method such as mechanical stirring method, ultrasonicdispersion method, high-pressure dispersion method, or media dispersionmethod, among which high-pressure dispersion method is preferablebecause the method makes it easy to disperse a filler more uniformly.The mixing order during such an operation may be any order as long as itcauses no particular problem such as generation of precipitate. Forinstance, the polymer and any other component such as a filler may be(i) added together to a medium and mixed with each other, (ii) added inany order to a medium and mixed with each other, or (iii) firstdissolved or dispersed in respective mediums and then mixed with eachother.

In a case where the medium for the coating solution contains water, itis preferable to perform a hydrophilization treatment on the firstporous layer before applying the coating solution to the first porouslayer. Performing a hydrophilization treatment on the first porous layerfurther improves the application property, and allows production of amore homogeneous second porous layer. A hydrophilization treatment isparticularly effective in a case where the medium contains water at ahigh concentration.

Examples of the hydrophilization treatment include (i) a, chemicaltreatment involving an acid, an alkali, or the like, (ii) a coronatreatment, and (iii) a plasma treatment.

Among the above hydrophilization treatments, a corona treatment ispreferable because it can not only hydrophilize the first porous layerwithin a relatively short time period, but also reform the polyolefinthrough corona discharge only at a surface and its vicinity of the firstporous layer and leave the inside of the first porous layer unchanged inquality while ensuring a high application property.

The medium is removed from the coating solution on the first porouslayer preferably by drying the first porous layer because drying issimple. Examples of the drying method include natural drying, blowdrying, drying by heating, and drying under reduced pressure, amongwhich drying by heating is preferable. Although depending on the mediumused, the drying temperature is preferably 30° C. to 80° C., morepreferably 50° C. to 80° C. A drying temperature of 30° C. or higherallows a sufficient drying speed. A drying temperature of 80° C. orlower allows deposition of a porous film having good appearance.

The following description discusses an example in which fine particlescontaining PVDF-based resin are used as a second porous layer. A methodfor producing a separator includes (i) a coating step of applying, toone surface or both surfaces of a first porous layer, an aqueousdispersion containing (a) fine particles containing PVDF-based resin and(b) a filler containing at least one of an organic compound and aninorganic compound; and (ii) a drying step of drying the aqueousdispersion which has been thus applied.

In a case where the method for producing the separator is thusconfigured, it is possible to produce the nonaqueous secondary batteryseparator of the present invention by evaporating a solvent in theaqueous dispersion. This makes it unnecessary to provide facilities forhandling organic solvents such as acetone which is generally used forproduction of separators, and therefore allows for a reduction inproduction cost for the separators. Therefore, it is possible to produceseparators with a high level of productivity.

[Coating Step]

In the coating step, an aqueous dispersion is applied to one surface orboth surfaces of a first porous layer so that an amount of fineparticles per layer is 0.1 g/m² to 6.0 g/m², said aqueous dispersion (i)containing (a) the fine particles containing PVDF-based resin and (b) afiller containing at least one of an organic compound and an inorganiccompound and (ii) being configured such that the filler is contained ata proportion of 20% or more by mass and 80% or less by mass with respectto a total mass of the fine particles and the filler.

[Aqueous Dispersion]

The aqueous dispersion is prepared by dispersing, suspending, oremulsifying, in/with a solvent, each of (i) fine particles containingPVDF-based resin and (ii) a filler containing at least one of an organiccompound and an inorganic compound, while the fine particles and thefiller are each in a solid state. The aqueous dispersion thus obtainedis to become a coating solution to be applied to a first porous layer.The aqueous dispersion may be an emulsion or a suspension.

The solvent for preparing the aqueous dispersion is at least water, andcan further include a solvent other than water.

The solvent other than water is not particularly limited, provided thatthe solvent does not dissolve the PVDF-based resin or the filler butallows each of the PVDF-based resin and the filler to be dispersed,suspended, or emulsified while being in a solid state. Examples of thesolvent include organic solvents such as: alcohols such as methanol,ethanol, and 2-propanol; acetone; tetrahydrofuran; methyl ethyl ketone;ethyl acetate; N-methylpyrrolidone; dimethylacetamide;dimethylformamide; and dimethylformamide.

An aqueous emulsion according to the present invention is an emulsionobtained by emulsifying, with water or with a mixture of water and theorganic solvent, (i) fine particles containing PVDF-based resin and (ii)a filler.

From environmental, safety, and economical viewpoints, it is preferableto use an aqueous emulsion obtained by emulsifying, with water or with amixture of water and alcohol, (i) fine particles containing PVDF-basedresin and (ii) a filler.

In terms of composition, the aqueous dispersion only needs to containwater, fine particles, and a filler. However, the aqueous dispersionpreferably contains them such that (i) a filler content is 20% or moreby mass and 80% or less by mass with respect to a total mass of the fineparticles and the filler or (ii) a volume of the filler is 15% or moreby volume and 90% or less by volume with respect to a total volume of avolume of the fine particles and a volume of the filler.

A publicly known thickening agent can be further contained, providedthat a viscosity suitable for coating can be ensured. A publicly knownsurface active agent can also be contained for improving dispersibilityof the fine particles and the filler in the aqueous dispersion.

The amount of the fine particles containing PVDF-based resin, which fineparticles are contained in the aqueous dispersion, is preferably 1% bymass to 50% by mass with respect to a total mass of the aqueousdispersion. By adjusting fine particle concentrations, it is possible toadjust the mass of the fine particles containing PVDF-based resin, whichfine particles are present in a nonaqueous secondary battery separator.

The aqueous dispersion may be applied to a first porous layer (forexample, polyolefin microporous film) through a conventional coatingmethod, for example, with use of a Meyer bar, a die coater, a reverseroll coater, a gravure coater, a micro-gravure coater, or a spray coat(spray coater). In a case where the fine particles containing PVDF-basedresin are fixed onto front and back surfaces of a first porous layer,the aqueous dispersion may be applied to one surface at a time and thendried. However, from the viewpoint of productivity, the aqueousdispersion is preferably applied simultaneously to both the surfaces ofthe first porous layer and then dried.

[Drying Step]

In the drying step, the aqueous dispersion, which has been applied tothe first porous layer in the coating step, is dried.

By drying the aqueous dispersion which has been applied to at least onesurface of the first porous layer (for example, polyolefin microporousfilm), a functional layer, which contains (i) an aggregate of fineparticles containing PVDF-based resin and (ii) a filler, is formed whilethe solvent in the aqueous dispersion is evaporated.

The fine particles containing PVDF-based resin, which fine particles areincluded in the functional layer after the drying step is taken,preferably retain a particle shape. As a result of the drying step beingtaken, the fine particles containing PVDF-based resin serve as a binder,so that an entire portion of the functional layer is integrally formedon the first porous layer such as a polyolefin microporous film.

[4. Nonaqueous Secondary Battery]

A nonaqueous secondary battery of the present invention achieves anelectromotive force through doping and dedoping with lithium. Thenonaqueous secondary battery of the present invention only needs toinclude a cathode, an anode, and the above-described separator, and isnot particularly limited in other arrangements. The nonaqueous secondarybattery includes (i) a battery element made of a structure (a) includingthe anode and the cathode facing each other via the above-describedseparator and (b) containing the electrolyte solution and (ii) anexterior member including the battery element. The nonaqueous secondarybattery is suitably applicable to a nonaqueous electrolyte secondarybattery, and is particularly applicable to a lithium ion secondarybattery. Note that the doping means storage, support, absorption, orinsertion, and means a phenomenon in which lithium ions enter an activematerial of the electrode (e.g., the cathode). A nonaqueous secondarybattery produced so as to include the above-described separator excelsin handling easiness of the separator, and thus has a high productionyield.

The cathode may be achieved as an active material layer which (i) isformed on a current collector and (ii) includes a cathode activematerial and a binder resin. The active material layer may furtherinclude a conductive auxiliary agent.

Examples of the cathode active material include a lithium-containingtransition metal oxide, specific examples of which include LiCoO₂,LiNiO₂, LiMn_(1/2)Ni_(1/2)O₂, LiCo_(1/3)Mn_(1/3)Ni_(1/3)O₂, LiMn₂O₄,LiFePO₄, LiCo_(1/2)Ni_(1/2)O₂, and LiAl_(1/4)Ni_(3/4)O₂.

Examples of the binder resin include a PVDF-based resin. Examples of theconductive auxiliary agent include carbon materials such as acetyleneblack, Ketjenblack, and graphite powder.

Examples of the current collector include aluminum foil, titanium foil,and stainless steel foil each having a thickness of 5 μm to 20 μm.

The anode may be achieved as an active material layer which (i) isformed on a current collector and (ii) includes an anode active materialand a binder resin. The active material layer may further include aconductive auxiliary agent. Examples of the anode active materialinclude a material capable of electrochemical storage of lithium.Specific examples of such a material include a carbon material; and analloy of (i) lithium and (ii) silicon, tin, aluminum, or the like.

Examples of the binder resin include a PVDF-based resin andstyrene-butadiene rubber. The separator of the present invention is ableto ensure sufficient adhesion to the anode even if the anode includesstyrene-butadiene rubber as the anode binder. Examples of the conductiveauxiliary agent include carbon materials such as acetylene black,Ketjenblack, and graphite powder.

Examples of the current collector include copper foil, nickel foil, andstainless steel foil each having a thickness of 5 μm to 20 μm. Insteadof the anode described above, metallic lithium foil may be employed asthe anode.

The electrolyte solution is a solution made of a nonaqueous solvent inwhich a lithium salt is dissolved. Examples of the lithium salt includeLiPF₆, LiBF₄, and LiClO₄.

Examples of the nonaqueous solvent include cyclic carbonate such asethylene carbonate, propylene carbonate, fluoroethylene carbonate, anddifluoroethylene carbonate; chain carbonate such as dimethyl carbonate,diethyl carbonate, ethyl methyl carbonate, and fluorine substituentsthereof; and cyclic ester such as γ-butyrolactone and γ-valerolactone.The present invention may use only (i) one kind of solvent or (ii) twoor more kinds of solvents in combination selected from the above.

The electrolyte solution is preferably the one obtained by (i) preparinga mixture through mixing of cyclic carbonate and chain carbonate at amass ratio (cyclic carbonate/chain carbonate) of 20/80 to 40/60 and (ii)dissolving in the mixture a lithium salt at a concentration of 0.5M to1.5M.

Examples of the exterior member include a metal can and a pack which ismade of an aluminum-laminated film. Examples of the shape of the batteryinclude a polygon, a cylinder, a coin shape.

It is possible to produce the nonaqueous secondary battery by, forexample, (i) causing the electrolyte solution to permeate the laminatedbody including the cathode, the anode, and the above-described separatorwhich is disposed between the cathode and the anode, (ii) causing thelaminated body to be accommodated in the exterior member (e.g., the packmade of the aluminum-laminated layer film), and (iii) pressing thelaminated body via the exterior member.

In a case where the PVDF-based resin is employed as the separator, sucha separator can be bonded to the electrode by stacking the separatoronto the electrode. Thus, although the above pressing is not anessential step for battery production in this case, it is preferable toperform the pressing in order to enhance adhesion between the electrodeand the separator. It is preferable to perform the pressing while theseparator and the electrode are heated (hot pressing) in order tofurther enhance adhesion between the electrode and the separator.

A manner how the separator is disposed between the cathode and the anodemay be (i) a manner (so-called stack system) in which at least onecathode, at least one separator, and at least one anode are stacked inthis order or (ii) a manner in which a cathode, a separator, an anode,and a separator are stacked in this order and the stack thus obtained isrolled up in a direction along a length of the stack.

Fourth Embodiment: Aspect 4 of Present Invention

The description below deals with a fourth embodiment of the presentinvention. The present invention is, however, not limited to such anembodiment. Further, the present invention is not limited to thedescription of the arrangements below, but may be altered in variousways by a skilled person within the scope of the claims. Any embodimentbased on a proper combination of technical means disclosed in differentembodiments is also encompassed in the technical scope of the presentinvention. In the present specification, any numerical range expressedas “A to B” means “not less than A and not greater than B” unlessotherwise stated.

[1. Nonaqueous Secondary Battery Separator]

The nonaqueous secondary battery separator is provided between a cathodeand an anode of a nonaqueous secondary battery, and includes (i) a filmyporous base material containing a polyolefin as a main component and(ii) a porous layer laminated on at least one surface of the porous basematerial.

Furthermore, in another embodiment of the present invention, thenonaqueous secondary battery separator may have, in addition to theporous layer, a heat-resistant layer made of heat-resistant resin. Theheat-resistant layer is preferably a layer containing aromaticpolyamide.

The following discusses the porous base material and the porous layerwhich constitute the nonaqueous secondary battery separator of thepresent invention.

[1-1. Porous Base Material]

The porous base material only needs to be made of a porous and filmybase material containing a polyolefin as a main component (apolyolefin-based porous base material), and is preferably a microporousfilm. Namely, the porous base material is preferably a porous film that(i) contains a polyolefin as a main component, (ii) has inside itselfpores connected to one another, and (iii) allows a gas, a liquid, or thelike to pass therethrough from one surface to the other. Also, theporous base material can be arranged such that, in a case where thebattery generates heat, the porous base material is melted so as to makea non-aqueous secondary battery separator non-porous. Thus, the porousbase material can impart to the non-aqueous secondary battery separatora shutdown function. The porous base material can be made of a singlelayer or a plurality of layers.

The porous base material has a porosity (D) of preferably 0.2 to 0.8 (20to 80% by volume), more preferably 0.3 to 0.75 (30 to 75% by volume), inorder to allow the separator to (i) retain a larger amount ofelectrolyte solution and (ii) achieve a function of reliably preventing(shutting down) the flow of an excessively large current at a lowertemperature. The porous base material has pores each having a pore sizeof preferably 3 μm or less, more preferably 1 μm or less, in order to,in a case where the porous base material is used as a separator, achievesufficient ion permeability and prevent particles from entering thecathode and the anode. Further, the porous base material has poreshaving an average pore size (hereinafter referred to also as “averagepore diameter (C)”), the average pore diameter (C) and porosity (D) ofthe porous base material satisfying the relation (C)/(D)≦0.13,preferably satisfying the relation (C)/(D)≦0.10. The average porediameter (C) of the porous base material has a value in ptm indicativeof the mean value of respective sizes of pores in the porous basematerial. The porosity (D) of the porous base material has a valueindicative of the proportion ((E)/(F)) of the volume (F) of void in theactual porous base material with reference to the volume (E) of theporous film when the porous base material is assumed to have no void.

The average pore diameter (C) of the porous base material is measuredwith use of a palm porometer available from PMI Co., Ltd. (model:CFP-1500A). The measurement involves, as a test liquid, GalWick (productname) available from PMI Co., Ltd., and is made of the following curves(i) and (ii) for the porous base material:

(i) Pressure-flow rate curve for the porous base material as immersed inthe test liquid

(ii) Pressure-flow rate curve, which is half the flow rate measured forthe dry porous base material

The average pore diameter (C) of the porous base material is calculatedby Formula (1) below on the basis of the value of a pressure Pcorresponding to the point of intersection of the curves (i) and (ii).

(C)=4 cos θr/P  (1)

In Formula (1) above, (C) represents the average pore diameter (μm), rrepresents the surface tension (N/m) of the test liquid, P representsthe above-mentioned pressure (Pa) corresponding to the point ofintersection, and θ represents the angle (°) of contact between thelaminated body and the test liquid.

The porosity (D) of the porous base material is measured through thefollowing method: A square piece with a 10 cm side is cut out from theporous base material. The weight W (g) and thickness E (cm) of the piececut out are then measured. The porosity (D) of the porous film iscalculated by Formula (2) below on the basis of (i) the weight (W) andthickness (E) measured above and (ii) the true specific gravity ρ(g/cm³) of the porous base material.

Porosity (D)=1−{(W/ρ)}/(10×10×E)  (2)

The average pore diameter (C) of the porous base material is controlledthrough, for example, a method of, in a case of reducing the porediameter, (i) uniformizing the dispersion state of a pore forming agentsuch as an inorganic filler or of a phase separating agent duringproduction of the porous base material, (ii) using, as a pore formingagent, an inorganic filler having smaller particle sizes, (iii) drawingthe porous base material in a state where the porous base materialcontains a phase separating agent, or (iv) drawing the porous basematerial at a low extension magnification. The porosity (D) of theporous base material is controlled through, for example, a method of, ina case of producing a porous base material having a high porosity, (i)increasing the amount of a pore forming agent such as an inorganicfiller or of a phase separating agent relative to the resin such as apolyolefin, (ii) drawing the porous base material after the phaseseparating agent has been removed, or (iii) drawing the porous basematerial at a high extension magnification.

The above average pore diameter (C)/porosity (D) of the porous basematerial should be a dominant factor in ease of infiltration of anelectrolyte solution into the polyolefin base material of a nonaqueouselectrolyte secondary battery separator including the porous basematerial.

A decrease in the value of (C)/(D) means (i) a decrease in the averagepore diameter (C) of the porous base material and/or (ii) an increase inthe porosity (D) of the porous base material.

A decrease in the average pore diameter (C) of the porous base materialshould increase the capillary force, which is presumed to serve as adriving force for introducing the electrolyte solution into pores insidethe polyolefin base material. Furthermore, smaller average pore diameter(C) can subdue generation of dendrites of lithium metal.

Further, an increase in the porosity (D) of the porous base materialshould decrease the volume of a portion of the polyolefin base materialwhich portion contains a polyolefin that cannot be permeated by theelectrolyte solution. This should be the reason why a decrease in thevalue of (C)/(D) described above increases the ease of infiltration ofan electrolyte solution into the polyolefin base material of anonaqueous electrolyte secondary battery separator including the porousbase material.

Specifically, in a case where, as described above, (C)/(D)≦0.13,desirably (C)/(D)≦0.10, it can increase the ease of infiltration of anelectrolyte solution into the polyolefin base material of a nonaqueouselectrolyte secondary battery separator with the porous base material sothat the ease of infiltration is sufficiently high for the separator tobe in actual use as a nonaqueous electrolyte secondary batteryseparator. Since the porous base material of the present invention haspores, the average pore diameter (C) of the porous film is larger than0. Accordingly, the value of (C)/(D) is larger than 0, too.

It is essential that the porous base material contains a polyolefincomponent at a proportion of 50% or more by volume with respect to wholecomponents contained in the porous base material. Such a proportion ofthe polyolefin component is preferably 90%, or more by volume, and morepreferably 95% or more by volume. The porous base material preferablycontains, as the polyolefin component, a high molecular weight componenthaving a weight-average molecular weight of 5×10⁵ to 15×10⁶. The porousbase material particularly preferably contains, as the polyolefincomponent, a polyolefin component having a weight-average molecularweight of 1,000,000 or more. This is because that (i) a porous basematerial containing such a polyolefin component and (ii) the whole of anonaqueous secondary battery including such a porous base materialachieve higher strength.

Examples of the polyolefin include high molecular weight homopolymers orcopolymers produced through polymerization of ethylene, propylene,1-butene, 4-methyl-1-pentene, 1-hexene, and/or the like. The porous basematerial can be a layer containing only one of these polyolefins and/ora layer containing two or more of these polyolefins. Among these, a highmolecular weight polyethylene containing ethylene as a main component isparticularly preferable. Note that the porous base material can containother component which is not a polyolefin, as long as the othercomponent does not impair the function of the porous base material.

The porous base material has an air permeability normally in a range of30 to 500 sec/100 cc, and preferably in a range of 50 to 300 sec/100 cc,in terms of Gurley values. A porous base material having an airpermeability within such a range achieves sufficient ion permeability ina case where the porous base material is used in the separator.

A thickness of the porous base material is selected as appropriate inview of the number of layers in the nonaqueous secondary battery.Particularly in a case where the porous layer is formed on one surface(or both surfaces) of the porous base material, the porous base materialhas a thickness of preferably 4 to 40 μm, and more preferably athickness of 7 to 30 μm.

The porous base material has a weight of normally 4 to 20 g/m², andpreferably 5 to 12 g/m². This is because a porous base material havingsuch a weight is possible to provide suitable strength, thickness,handling easiness, and weight of the laminated body and is also possibleto enhance a weight energy density and/or a volume energy density in acase where the porous base material is used in the separator of thenonaqueous secondary battery.

Suitable examples of such a porous base material include a porouspolyolefin layer disclosed in Japanese Patent Application Publication,Tokukai, No. 2013-14017 A, a polyolefin porous film disclosed inJapanese Patent Application Publication, Tokukai, No. 2012-54229 A, anda polyolefin base material porous film disclosed in Japanese PatentApplication Publication, Tokukai, No. 2014-040580 A.

The porous base material may be produced, through any publicly-knowntechnique, and is not particularly limited to any specific method. Forexample, as disclosed in Japanese Patent Application Publication,Tokukaihei, No. 7-29563 A (1995), the porous base material may beproduced through a method of (i) adding a plasticizing agent to athermoplastic resin to shape the thermoplastic resin into a film andthen (ii) removing the plasticizing agent with use of an appropriatesolvent.

Specifically, in a case of, for example, producing a porous basematerial with use of a polyolefin resin containing (i) an ultra highmolecular weight polyethylene and (ii) a low molecular weight polyolefinhaving a weight-average molecular weight of 10,000 or less, such aporous base material is, in terms of production cost, preferablyproduced through the method including the steps of:

(1) kneading (i) 100 parts by weight of the ultra high molecular weightpolyethylene, (ii) 5 to 200 parts by weight of the low molecular weightpolyolefin having a weight-average molecular weight of 10,000 or less,and (iii) 100 to 400 parts by weight of an inorganic filler of calciumcarbonate or the like to produce a polyolefin resin composition,

(2) shaping the polyolefin resin composition into a sheet,

(3) removing the inorganic filler from the sheet produced in the step(2), and

(4) drawing the sheet produced in the step (3) to produce an A layer.

Alternatively, the porous base material, may be produced through any ofthe methods explained in the above-described Patent Literatures.

The porous base material may alternatively be a commercially availableproduct having the above physical properties.

[1-2. Porous Layer]

The porous layer of the present invention contains polyvinylidenefluoride-based resin (PVDF-based resin). The porous layer (i) has insideitself pores connected to one another, and (ii) allows a gas or a liquidto pass therethrough from one surface to the other. Also, in the presentembodiment, the porous layer is provided on one surface of the porousbase material to be an outermost layer of the separator and to be alayer capable of contacting an electrode.

The inventors of the present invention have diligently studied and foundthat in a case where the porous layer is immersed, at 25° C. for 24hours, in an electrolyte solution obtained by dissolving LiPF₆ with aconcentration of 1.0 mol/liter in a mixed solvent of ethylmethylcarbonate, diethylcarbonate, and ethylenecarbonate in a volume ratio of50:20:30, and then the volume of the porous layer (resin having absorbedthe electrolyte solution) per one square meter of the porous layer iscontrolled to be 0.05 to 5.00 cm³, it is possible to subdue a decreasein cycle characteristics while maintaining adhesiveness between theporous layer and an electrode.

In particular, the inventors of the present invention have found that bycombining (i) the porous base material satisfying the relation ofaverage pore diameter (C)/porosity (D)≦0.13 with (ii) the porous layerin which after immersion of the porous layer in the electrolytesolution, the resin having absorbed the electrolyte solution has avolume of 0.05 to 5.00 cm³ per square meter of the porous layer, it ispossible to subdue a decrease in cycle characteristics while maintainingadhesiveness between the separator and an electrode. That is, in a caseof a nonaqueous electrolyte secondary battery separator having astructure in which a polyolefin-based porous base material and a porouslayer containing polyvinylydene fluoride-based resin are laminated, bysatisfying the conditions of (i) average pore diameter (C)/porosity(D)≦0.13 and (ii) the resin having a volume of 0.05 to 5.00 cm³ persquare meter of the porous layer after being immersed in the electrolytesolution, the nonaqueous electrolyte secondary battery separator cansubdue the decrease in cycle characteristics while maintainingadhesiveness between the nonaqueous electrolyte secondary batteryseparator and an electrode.

Furthermore, by combining (i) the porous base material satisfying therelation of average pore diameter (C)/porosity (D)≦0.13 with (ii) theporous layer in which after immersion of the porous layer in theelectrolyte solution, the resin having absorbed the electrolyte solutionhas a volume of 0.05 to 5.00 cm³ per square meter of the porous layer,it is possible to further subdue generation of dendrides and increase ashutdown property.

As described above, by setting the average pore diameter (C) of theporous base material to be smaller so as to satisfy the relation of(C)/(D)≦0.13, it is possible to subdue generation of dendrites in theporous base material due to lithium metal. Furthermore, by using theporous layer whose volume of resin having absorbed the electrolytesolution is not more than 5.00 cm³, the resin (gel) constituting theporous layer can maintain suitable hardness when the resin absorbs theelectrolyte solution, thereby subduing generation of dendrites. In acase where the resin having absorbed the electrolyte solution has avolume of more than 5.00 cm³, the gel constituting the porous layerbecomes excessively soft, making it difficult to subdue generation ofdendrites. As described above, in the present embodiment, both of theporous base material and the porous layer allow subduing generation ofdendrites. Furthermore, since the resin (gel) which constitutes theporous layer and which has absorbed the electrolyte solution maintainssuitable hardness, the gel is less likely to enter into pores of theporous base material satisfying the relation of (C)/(D)≦0.13. This canprevent the shutdown property from decreasing due to the gel enteringinto pores of the porous base material. This allows improving theshutdown property.

Herein, the electrolyte solution obtained by dissolving LiPF₆ of 1.0mol/liter in concentration in a mixed solvent of ethylmethyl carbonate,diethylcarbonate, and ethylenecarbonate in a volume ratio of 50:20:30 isan example of an electrolyte solution used in a nonaqueous secondarybattery. Accordingly, the porous layer immersed in the electrolytesolution at 25° C. for 24 hours simulates the porous layer built in thenonaqueous secondary battery.

Furthermore, the resin having absorbed the electrolyte solution is in astate where the resin constituting the porous layer is swollen due tothe electrolyte solution, i.e. a state where the resin is gelatinized.

Furthermore, the volume (basis volume) of resin having absorbed theelectrolyte solution per square meter of the porous layer was measuredas follows.

1) Calculation of Weight Increased after Swelling Due to ElectrolyteSolution

Polyvinylidene fluoride-based resin was applied to an aluminum cup, andwas dried in a vacuum at 120° C. for 8 hours. The filmy non-porouspolyvinylidene fluoride-based resin thus obtained was cut into a piecehaving a size of 2 cm², and a weight W1 of the sample was measured. Thesample was immersed, at 25° C. for 24 hours, in an electrolyte solutionobtained by dissolving LiPF₆ with a concentration of 1.0 mol/liter in amixed solvent of ethylmethyl carbonate, diethylcarbonate, andethylenecarbonate in a volume ratio of 50:20:30. Then, the sample wastaken out and a weight W2 thereof was measured. A weight increased afterswelling of the sample was calculated based on the formula below.

Weight increased after swelling W2′=W2−W1

where W1 represents the weight of the sample before it was immersed, andW2 represents the weight of the sample after immersion of 24 hours.

2) Calculation of Degree of Swelling in Volume of Resin Having SwollenDue to Electrolyte Solution

The degree of swelling in volume of polyvinylidene fluoride-based resinhaving swollen due to electrolyte solution was calculated based on theformula below.

The degree of swelling in volume=(W1/ρ1+W2′/ρ2)/(W1/ρ1)

where ρ1 represents specific gravity of PVDF-based resin at 25° C., andρ2 represents specific gravity of an electrolyte solution obtained bydissolving LiPF₆ with a concentration of 1.0 mol/liter in a mixedsolvent of ethylmethyl carbonate, diethylcarbonate, andethylenecarbonate in a volume ratio of 50:20:30.

3) Calculation of Basis Volume of Porous Layer after Swelling Due toElectrolyte Solution

Basis weight (weight per 1 square meter) Wd of the porous layer in adried state was measured and the basis weight was divided by thespecific gravity of the PVDF-based resin at 25° C., so that basis volume(volume per 1 square meter) Vd of the resin component of the porouslayer in a dried state was measured.

The basis volume Vd of the resin component of the porous layer in adried state is multiplied by the degree of swelling in volume of theresin after swelling due to the electrolyte solution, so that basisvolume Vw of the porous layer after swelling due to the electrolytesolution (i.e. volume of resin having absorbed the electrolyte solutionper square meter of the porous layer) can be obtained.

By controlling the resin having absorbed the electrolyte solution persquare meter of the porous layer immersed in the electrolyte solution tohave a volume of 0.05 cm³ or more, it is possible to secure adhesivenessbetween the porous layer and the electrode. That is, in a case where inthe porous layer after being immersed in the electrolyte solution, theresin having absorbed the electrolyte solution has a volume of less than0.05 cm³ per square meter of the porous layer, the amount of gelatinizedresin is small and consequently it is difficult to maintain adhesivenessbetween the porous layer and the electrode. In contrast, in a case wherethe resin having absorbed the electrolyte solution per square meter ofthe porous layer has a volume of not less than 0.05 cm³, it is possibleto secure adhesiveness between the porous layer and the electrode. Theresin having absorbed the electrolyte solution per square meter of theporous layer more preferably has a volume of not less than 0.25 cm³.

Furthermore, by controlling the resin having absorbed the electrolytesolution per square meter of the porous layer immersed in theelectrolyte solution to have a volume of not more than 5.00 cm³, it ispossible to improve the cycle characteristics of the nonaqueoussecondary battery including the porous layer. That is, in a case wherethe resin having absorbed the electrolyte solution per square meter ofthe porous layer has a volume of more than 5.00 cm³, the gelatinizedporous layer has an increased resistance in transmissivity of ions. Incontrast, in a case where the resin having absorbed the electrolytesolution has a volume of not more than 5.00 cm³ per square meter of theporous layer, it is possible to subdue a decrease in mobility of ions inthe gelatinized porous layer and to subdue an increase in charging time.Consequently, it is possible to subdue oxidation and decomposition ofthe electrolyte solution at a cathode side and deposition of metal at ananode side, thereby improving the cycle characteristics.

The resin having absorbed the electrolyte solution per square meter ofthe porous layer more preferably has a volume of not more than 1.50 cm³.

Furthermore, the porous layer of the present invention after beingimmersed in the electrolyte solution at 25° C. for 24 hours preferablyhas a porosity of 0.5 to 55.0%.

Here, the porosity of the porous layer after being immersed in theelectrolyte solution for 24 hours can be calculated as follows. First, avolume A of the porous layer after being immersed in the electrolytesolution for 24 hours and including voids is measured. Then, a weight ofthe porous layer after being immersed in the electrolyte solution for 24hours is measured, and the weight is divided by true density of resinhaving absorbed the electrolyte solution, so as to obtain a volume B ofresin (resin gelatinized by the electrolyte solution) itself. Then, theporosity is calculated in accordance with porosity=100×(A−B)/A.

In a case where the nonaqueous secondary battery includes the porouslayer and the porous layer gets gelatinized due to the electrolytesolution, the electrolyte solution in the gel has smaller dispersionratio. Consequently, in a case where the electrolyte solution is driedup locally in the porous layer due to some influence, it is impossibleto supply the electrolyte solution to the portion where the electrolytesolution is dried up. This maintains a state where the electrolytesolution is dried up, thereby causing a decrease in the cyclecharacteristics.

However, by controlling the porous layer after being immersed in theelectrolyte solution for 24 hours to have a porosity of 0.005 (0.5% byvolume) or more, the porous layer built in the nonaqueous secondarybattery has the electrolyte solution in the form of a liquid in voids ofthe porous layer. Accordingly, even if the electrolyte solution is driedup locally, the electrolyte solution in a neighboring void is suppliedto the portion where the electrolyte solution is dried up. This preventsa state where the electrolyte solution is dried up from beingmaintained, thereby subduing a decrease in the cycle characteristics.

Furthermore, by controlling the porous layer after being immersed in theelectrolyte solution for 24 hours to have a porosity of 0.55 (55% byvolume) or less, it is possible to maintain strength of the gelatinizedporous layer and secure an area of the gelatinized porous layer whichcontacts an electrode when built in the nonaqueous secondary battery.This allows preventing a decrease in adhesiveness between the porouslayer and the electrode. This allows preventing a decrease in capacitydue to an increase in distance between a cathode and an anode in acharge/discharge cycle.

Furthermore, the porous layer of the present invention preferably has anaverage pore diameter of 0.8 to 95.0 nm after the porous layer isimmersed in the electrolyte solution at 25° C. for 24 hours.

The average pore diameter of the porous layer after being immersed inthe electrolyte solution for 24 hours can be measured with a scanningprobe microscope (SPM).

In the nonaqueous secondary battery, there is a possibility that aslight amount of water content having entered the nonaqueous secondarybattery generates an inorganic solid. When such an inorganic solid clogspores in the porous layer, the capacity of the battery decreases.

However, by controlling the porous layer after being immersed in theelectrolyte solution for 24 hours to have an average pore diameter of0.8 nm or more, it is possible to reduce a probability of such aninorganic solid clogging the pores, thereby preventing a decrease incapacity of the battery.

Furthermore, by controlling the porous layer after being immersed in theelectrolyte solution for 24 hours to have an average pore diameter of95.0 nm or less, it is possible to maintain strength of the gelatinizedporous layer and secure an area of the gelatinized porous layer whichcontacts an electrode when built in the nonaqueous secondary battery.This allows maintaining adhesiveness between the porous layer and theelectrode. This allows preventing a decrease in capacity due to anincrease in distance between a cathode and an anode in acharge/discharge cycle.

It is preferable that the resin constituting the porous layer containspolyvinylidene fluoride-based resin and has a structure in whichskeletons with a diameter of 1 μm or less are connected with each otherto form a three-dimensional network.

Examples of the polyvinylidene fluoride-based resin include homopolymersof vinylidene fluoride (i.e., polyvinylidene fluoride); copolymers(e.g., polyvinylidene fluoride copolymer) of vinylidene fluoride andother monomer(s) polymerizabie with vinylidene fluoride; and mixtures ofthese polymers. Examples of the monomer polymerizable with vinylidenefluoride include hexafluoropropylene, tetrafluoroethylene,trifluoroethylene, trichloroethylene, and vinyl fluoride. The presentinvention can use (i) one kind of monomer or (ii) two or more kinds ofmonomers selected from above. The polyvinylidene fluoride-based resincan be synthesized through emulsion polymerization or suspensionpolymerization.

The polyvinylidene fluoride-based resin preferably contains vinylidenefluoride at a proportion of 95 mol % or more (more preferably, 98 mol %or more). A polyvinylidene fluoride-based resin containing vinylidenefluoride at a proportion of 95 mol % or more is more likely to allow theporous layer to achieve a mechanical strength and a heat resistanceagainst a pressure or heat occurred in battery production.

The porous layer preferably contains two kinds of polyvinylidenefluoride-based resins (a first resin and a second resin below) that aredifferent from each other in a content of hexafluoropropylene.

The first resin is (i) a vinylidene fluoride-hexafluoropropylenecopolymer containing hexafluoropropylene at a proportion of more than 0mol % and 1.5 mol % or less or (ii) a vinylidene fluoride homopolymer(containing hexafluoropropylene at a proportion of 0 mol %).

The second resin is a vinylidene fluoride-hexafluoropropylene copolymercontaining hexafluoropropylene at a proportion of more than 1.5 mol %.

The porous layer containing the two kinds of polyvinylidenefluoride-based resins is adhered to the electrode more favorably, ascompared with a porous layer not containing one of the two kinds ofpolyvinylidene fluoride-based resins. Further, the porous layercontaining the two kinds of polyvinylidene fluoride-based resins hasimproved adhesiveness to the porous base material and is separated fromthe porous base material more favorably, as compared with a porous layernot containing one of the two kinds of polyvinylidene fluoride-basedresins. The first resin and the second resin are preferably mixed at amixing ratio (mass ratio, first resin:second resin) of 15:85 to 85:15.

The polyvinylidene fluoride-based resin has a weight-average molecularweight of preferably 300,000 to 3,000,000. A polyvinylidenefluoride-based resin having a weight-average molecular weight of 300,000or more allows the porous layer to attain a mechanical property withwhich the porous layer can endure a process for adhering the porouslayer to the electrode, thereby allowing the porous layer and theelectrode to adhere to each other sufficiently. Meanwhile, apolyvinylidene fluoride-based resin having a weight-average molecularweight of 3,000,000 or less does not cause the coating solution, whichis to be applied in order to shape the porous layer, to have a too highviscosity, which allows the coating solution to have excellent shapingeasiness. The weight-average molecular weight of the polyvinylidenefluoride-based resin is more preferably 300,000 to 2,000,000, andfurther preferably 500,000 to 1,500,000.

The polyvinylidene fluoride-based resin has a fibril diameter ofpreferably 10 nm to 1000 nm, in terms of the cycle characteristic.

The porous layer may contain other resin which is not the polyvinylidenefluoride-based resin. Examples of the other resin includestyrene-butadiene copolymer; homopolymers or copolymers of vinylnitriles such as acrylonitrile and methacrylonitrile; and polyetherssuch as polyethylene oxide and polypropylene oxide.

Further, the porous layer may contain a filler made of inorganic matteror organic matter. A porous layer containing the filler can improveslidability and/or heat resistance of the separator. The filler may bean organic filler or an inorganic filler each of which is stable in anonaqueous electrolyte solution and is electrochemically stable. Thefiller preferably has a heat-resistant temperature of 150° C. or more toensure safety of the battery.

Examples of the organic filler include crosslinked high molecule fineparticles such as crosslinked polyacrylic acid, crosslinked polyacrylicacid ester, crosslinked polymethacrylic acid, crosslinkedpolymethacrylic acid ester, crosslinked polymethyl methacrylate,crosslinked polysilicone, crosslinked polystyrene, crosslinkedpolydivinyl benzene, a crosslinked product of a styrene-divinylbenzenecopolymer, polyimide, a melamine resin, a phenol resin, abenzoguanamine-formaldehyde condensate; and heat-resistant high moleculefine particles such as polysulfone, polyacrylonitrile, polyaramid,polyacetal, and thermoplastic polyimide.

A resin (high molecule) contained in the organic filler may be amixture, a modified product, a derivative, a copolymer (a randomcopolymer, an alternating copolymer, a block copolymer, or a graftcopolymer), or a crosslinked product of any of the molecules listedabove as examples.

Examples of the inorganic filler include metal hydroxides such asaluminum hydroxide, magnesium hydroxide, calcium hydroxide, chromiumhydroxide, zirconium hydroxide, nickel hydroxide, and boron hydroxide;metal oxides such as alumina and zirconia; carbonates such as calciumcarbonate and magnesium carbonate; sulfates such as barium sulfate andcalcium sulfate; clay minerals such as calcium silicate and talc. Amongthese, the inorganic filler is preferably a metal hydroxide, in terms ofachievement of fire retardance and/or electricity removal effects.

The present invention may use (i) only a single filler or oii) two ormore of fillers in combination.

The filler has a volume average particle size of preferably 0.01 μm to10 μm, in order to ensure (i) fine adhesion and fine slidability and(ii) shaping easiness of the separator. A lower limit of the volumeaverage particle size is more preferably 0.1 μm or more, whereas anupper limit of the volume average particle size is more preferably 5 μmor less.

The filler is constituted by particles of any shape, which may be asphere, an ellipse, a plate-shape, a bar-shape, or an irregular shape.In order to prevent a short circuit in the battery, the particles arepreferably (i) plate-shaped particles or (ii) primary particles whichare not aggregated.

The filler forms fine bumps on a surface of the porous layer, therebyimproving the slidability. A filler constituted by (ii plate-shapedparticles or (ii) primary particles which are not aggregated forms finerbumps on the surface of the porous layer, so that the porous layer isadhered to the electrode more favorably.

The porous layer contains the filler at a proportion of preferably 1% bymass to 30% by mass with respect to a total amount of the polyvinylidenefluoride-based resin and the filler. A porous layer containing thefiller at a proportion of 1% or more by mass is likely to exhibit theeffect of forming fine bumps on the surface of the porous layer so as toimprove the slidability of the separator. From this viewpoint, theporous layer contains the filler more preferably at a proportion of 3%or more by mass. Meanwhile, a porous layer containing the filler at aproportion of 30% or less by mass allows the porous layer to maintainmechanical strength. With this arrangement, for example, during aprocess for producing an electrode body by rolling up a stack of theelectrode and the separator, the separator is hardly cracked and/or thelike. From this viewpoint, the porous layer contains the filler at aproportion of more preferably 20% or less by mass, and furtherpreferably 10% or less by mass.

In order to prevent, in a process of slitting the separator, a slittedsurface of the separator from becoming fibrous, bending, and/orpermitting intrusion of scraps occurred as a result of the slitting, theporous layer contains the filler at a proportion of preferably 1% ormore by mass, and more preferably 3% or more by mass, with respect to atotal amount of the polyvinylidene fluoride-based resin and the filler.

In order to ensure adhesion to the electrode and a high energy density,the porous layer has, on one surface of the porous base material, anaverage thickness of preferably 0.5 μm to 10 μm, and more preferably 1μm to 5 μm.

The porous layer is preferably made porous sufficiently, in terms of ionpermeability. Specifically, the porous layer has a porosity ofpreferably 30% to 60%. The porous layer has an average pore size of 20nm to 100 nm.

The porous layer has a surface roughness, in terms of a ten-pointaverage roughness (Rz), of preferably 0.8 pin to 8.0 μm, more preferably0.9 μm to 6.0 μm, and further preferably 1.0 μm to 3.0 μm. The ten-pointaverage roughness (Rz) is a value measured by a method according to JISB 0601-1994 (or Rzjis of JIS B 0601-2001). Specifically, “Rz” is a valuemeasured by ET4000 (available from Kosaka Laboratory Ltd.) with ameasurement length of 1.25 mm, a measurement rate of 0.1 mm/sec, and atemperature and humidity of 25° C./50% RH.

The porous layer has a coefficient of kinetic friction of preferably 0.1to 0.6, more preferably 0.1 to 0.4, and further preferably 0.1 to 0.3.The coefficient of kinetic friction is a value measured by a methodaccording to JIS K7125. Specifically, a coefficient of kinetic frictionin the present invention is a value measured by Surface Property Tester(available from Heidon).

An applied amount of the porous layer is, on one surface of the porousbase material, preferably 0.5 g/m² to 1.5 g/m² in terms of adhesion tothe electrode and ion permeability.

[2. Method for Producing Nonaqueous Secondary Battery Separator]

A method for producing the nonaqueous secondary battery separator of thepresent invention is not limited to any particular one, but may beselected from various methods, provided that the nonaqueous secondarybattery separator can be obtained.

The nonaqueous secondary battery separator is produced by forming, on asurface of a polyolefin resin fine porous film as a porous basematerial, a porous layer containing a polyvinylidene fluoride-basedresin through, for example, any one of methods (1) to (3) below.

(1) Method of (i) applying to a surface of the porous base material asolution in which a resin for forming the porous layer is dissolved andthen (ii) immersing the resulting porous base material into a depositionsolvent as a poor solvent for the resin to deposit a porous layercontaining the resin

(2) Method of (i) applying to a surface of the porous base material asolution in which a resin for forming the porous layer is dissolved andthen (ii) making the solution acidic with use of low-boiling organicacid to deposit a porous layer containing the resin

(3) Method of (i) applying to a surface of the porous base material asolution in which a resin for forming the porous layer is dissolved andthen (ii) evaporating the solvent in the solution by far infraredheating or freeze drying to deposit a porous layer containing the resin

The methods (1) and (2) may each further involve a step of, after theporous layer has been deposited, drying the laminated body produced.

In a case where in any of the above methods, the resin for forming theporous layer is, for example, a PVDF-based resin, the solvent in whichthe resin is dissolved is preferably N-methylpyrrolidone.

In a case where in the method (1), the resin for forming the porouslayer is, for example, a PVDF-based resin, the solvent for depositingthe porous layer is preferably isopropyl alcohol or t-butyl alcohol.

Furthermore, the porous layer produced by the method (1) may beirradiated with an electron ray (EB: Electric Beam). This allowsincreasing crosslinked resin in the porous layer.

In the method (2), the organic acid is, for example, paratoluenesulfonic acid, acetic acid etc.

In the method (3), far infrared eating and freeze drying areadvantageous over other drying methods (such as air drying) in that therespective shapes of pores in the porous layer are not easily changeableduring the deposition.

In a case of producing a laminated body further including aheat-resistant layer, such a heat-resistant layer may be depositedthrough a method similar to the above method except that the resin forforming a porous layer is replaced with a resin for forming aheat-resistant layer.

To form a porous layer containing a filler, the filler may be dispersedin the solution in which the resin for forming the porous layer isdissolved.

In the present embodiment, in any of the methods (1) to (3), by varyingan amount of resin in the solution in which the resin for forming theporous layer is dissolved, it is possible to adjust a volume of resinhaving absorbed the electrolyte solution per square meter of the porouslayer after being immersed in the electrolyte solution.

Furthermore, by varying an amount of the solvent in which the resin forforming the porous layer is dissolved, it is possible to adjust aporosity and an average pore diameter of the porous layer after beingimmersed in the electrolyte solution.

[3. Nonaqueous Secondary Battery]

A nonaqueous secondary battery of the present invention achieves anelectromotive force through doping and dedoping with lithium. Thenonaqueous secondary battery of the present invention only needs toinclude a laminated body in which a cathode, an anode, and theabove-described nonaqueous secondary battery separator of the presentinvention are laminated, and is not particularly limited in otherarrangements. The nonaqueous secondary battery includes (i) a batteryelement made of a structure (a) including the anode and the cathodefacing each other via the above-described nonaqueous secondary batteryseparator and (b) containing the electrolyte solution and (ii) anexterior member including the battery element. The nonaqueous secondarybattery is suitably applicable to a nonaqueous electrolyte secondarybattery, and is particularly applicable to a lithium ion secondarybattery. Note that the doping means storage, support, absorption, orinsertion, and means a phenomenon in which lithium ions enter an activematerial of the electrode (e.g., the cathode). A nonaqueous secondarybattery produced so as to include the above-described nonaqueoussecondary battery separator of the present invention excels in handlingeasiness of the separator, and thus has a high production yield.

The cathode may be achieved as an active material layer which (i) isformed on a current collector and (ii) includes a cathode activematerial and a binder resin. The active material layer may furtherinclude a conductive auxiliary agent.

Examples of the cathode active material include a lithium-containingtransition metal oxide, specific examples of which include LiCoO₂,LiNiO₂, LiMn_(1/2)Ni_(1/2)O₂, LiCo_(1/3)Mn_(1/3)Ni_(1/3)O₂, LiMn₂O₄,LiFePO₄, LiCo_(1/2)Ni_(1/2)O₂, and LiAl_(1/4)Ni_(3/4)O₂.

Examples of the binder resin include a polyvinylidene fluoride-basedresin. Examples of the conductive auxiliary agent include carbonmaterials such as acetylene black, Ketjenblack, and graphite powder.

Examples of the current collector include aluminum foil, titanium foil,and stainless steel foil each having a thickness of 5 μm to 20 μm.

The anode may be achieved as an active material layer which (i) isformed on a current collector and (ii) includes an anode active materialand a binder resin. The active material layer may further include aconductive auxiliary agent. Examples of the anode active materialinclude a material capable of electrochemical storage of lithium.Specific examples of such a material include a carbon material; and analloy of (i) lithium and (ii) silicon, tin, aluminum, or the like.

Examples of the binder resin include a polyvinylidene fluoride-basedresin and styrene-butadiene rubber. The separator of the presentinvention is able to ensure sufficient adhesion to the anode even if theanode includes styrene-butadiene rubber as the anode binder. Examples ofthe conductive auxiliary agent include carbon materials such asacetylene black, Ketjenblack, and graphite powder.

Examples of the current collector include copper foil, nickel foil, andstainless steel foil each having a thickness of 5 μm to 20 μm. Insteadof the anode described above, metallic lithium foil may be employed asthe anode.

The electrolyte solution is a solution made of a nonaqueous solvent inwhich a lithium salt is dissolved. Examples of the lithium salt includeLiPF₆, LiBF₄, and LiClO₄.

Examples of the nonaqueous solvent include all solvents normally used ina nonaqueous secondary battery, and are not limited to the above mixedsolvent (ethyl methyl carbonate, diethyl carbonate, and ethylenecarbonate in volume ratio of 50:20:30)

Examples of the nonaqueous solvent include cyclic carbonate such asethylene carbonate, propylene carbonate, fluoroethylene carbonate, anddifluoroethylene carbonate; chain carbonate such as dimethyl carbonate,diethyl carbonate, ethyl methyl carbonate, and fluorine substituentsthereof; and cyclic ester such as γ-butyrolactone and γ-valerolactone.The present invention may use only (i) one kind of solvent or (ii) twoor more kinds of solvents in combination selected from the above.

The electrolyte solution is preferably the one obtained by (i) preparinga mixture through mixing of cyclic carbonate and chain carbonate at amass ratio (cyclic carbonate/chain carbonate) of 20/80 to 40/60 (morepreferably 30/70) and (ii) dissolving in the mixture a lithium salt at aconcentration of 0.5M to 1.5M.

Examples of the exterior member include a metal can and a pack which ismade of an aluminum-laminated film. Examples of the shape of the batteryinclude a square, a rectangular, a cylinder, a coin shape.

It is possible to produce the nonaqueous secondary battery by, forexample, (i) causing the electrolyte solution to permeate the laminatedbody including the cathode, the anode, and the above-described separatorwhich is disposed between the cathode and the anode, (ii) causing thelaminated body to be accommodated in the exterior member (e.g., the packmade of the aluminum-laminated layer film), and (iii) pressing thelaminated body via the exterior member.

In a case where the polyvinylidene fluoride-based resin is employed asthe separator, such a separator can be bonded to the electrode bystacking the separator onto the electrode. Thus, although the abovepressing is not an essential step for battery production in this case,it is preferable to perform the pressing in order to enhance adhesionbetween the electrode and the separator. It is preferable to perform thepressing while the separator and the electrode are heated (hot pressing)in order to further enhance adhesion between the electrode and theseparator.

A manner how the separator is disposed between the cathode and the anodemay be (i) a manner (so-called stack system) in which at least onecathode, at least one separator, and at least one anode are stacked inthis order or (ii) a manner in which a cathode sheet, a separator, ananode sheet, and a separator are stacked in this order and the stackthus obtained is rolled up in a direction along a length of the stack.

Another Embodiment

The above description has dealt with a case where a nonaqueous secondarybattery separator in which a porous layer is formed on a porous basematerial is produced, and a cathode sheet and an anode sheet arelaminated between which the nonaqueous secondary battery separator issandwiched, so that a laminate including the nonaqueous secondarybattery separator and electrodes is produced. However, the method forproducing the nonaqueous secondary battery of the present invention isnot limited to this case.

For example, a porous layer may be formed by applying, to at least onesurface of a cathode sheet or an anode sheet, a solution in which aresin for forming the porous layer is dissolved. This method for formingthe porous layer may be one of the above methods (1) to (3). Then, thecathode sheet and the anode sheet are laminated between which a porousbase material is sandwiched, and the resulting laminate is thermallypressed, so as to produce a laminate including a nonaqueous secondarybattery separator and electrodes. In this case, the electrode sheet onwhich the porous layer is formed may be provided so that the porouslayer faces the porous base material. This allows producing a laminatein which the electrode, the porous layer, the porous base material,(porous layer), and the electrode are laminated. Consequently, theporous layer is provided between the electrode and the porous basematerial, so that the cycle characteristics can be improved whilemaintaining adhesiveness between the porous base material and theelectrode.

EXAMPLES Examples of Aspect 1 of Present Invention

(Average Pore Diameter (C) of Porous Film)

The average pore diameter C) was measured with use of a palm porometeravailable from PMI Co., Ltd. (model: CFP-1500A). The measurementinvolved, as a test liquid, GalWick (product name) available from PMICo., Ltd. and was made of the following curves (i) and (ii) for theporous film:

(i) Pressure-flow rate curve for the porous film as immersed in the testliquid

(ii) Pressure-flow rate curve, which is half the flow rate measured forthe dry porous film

The average pore diameter (C) of the porous film was calculated byFormula (3) below on the basis of the value of a pressure Pcorresponding to the point of intersection of the curves (i) and (ii).

(C)=4 cos θr/P  (3)

In Formula (3) above, (C) represents the average pore diameter (μm), rrepresents the surface tension (N/m) of the test liquid, P representsthe above-mentioned pressure (Pa) corresponding to the point ofintersection, and θ represents the angle (°) of contact between theporous film and the test liquid.

(Porosity (D) of Porous Film)

A square piece with a 10 cm side was cut out from the porous film. Theweight W (g) and thickness E (cm) of the piece cut out were thenmeasured. The porosity (D) of the porous film was calculated by Formula(4) below on the basis of (i) the weight (W) and thickness (E) measuredabove and (ii) the true specific gravity ρ (g/cm³) of the porous film.

Porosity (D)=1−{(W/ρ)}/(10×10×E)  (4)

Production of Separator Example 1

A vinylidene fluoride-hexafluoropropylene (PVDF-HEP) copolymer wasdissolved in N-methylpyrrolidone at a concentration of 7% by weight toprepare a coating solution. This coating solution was applied to onesurface of a polyethylene porous film (base material) having a thicknessof 17 μm and the average pore diameter (C) and porosity (D) shown inTable 1 below, where (C)/(D)=0.08. The polyethylene porous film was thenimmersed in isopropyl alcohol to deposit a porous layer containing apolyvinylidene fluoride-based resin (PVDF-HEP copolymer). Thepolyethylene porous film, on a surface of which the porous layer wasdeposited, was then dried to produce a laminated body including (i) apolyethylene porous film and (ii) a porous layer on a surface of thepolyethylene porous film, the porous layer containing a polyvinylidenefluoride-based resin (PVDF-HEP copolymer).

Example 2

An operation was conducted as in Example 1 except that the PVDF-HEPcopolymer was replaced with a polyvinylidene fluoride (PVDF) resin toproduce a laminated body including (i) a polyethylene porous film and(ii) a porous layer on a surface of the polyethylene porous film, theporous layer containing PVDF.

Example 3

An operation was conducted as in Example 1 except that the PVDF-HEPcopolymer was replaced with CHEMIPEARL W401, a polyolefin aqueousdispersion available from Mitsui Chemicals, Inc. to produce a laminatedbody including (i) a polyethylene porous film and (ii) a porous layer ona surface of the polyethylene porous film, the porous layer containingCHEMIPEARL W401, a polyolefin aqueous dispersion available from MitsuiChemicals, Inc.

Example 4

An operation was conducted as in Example 1 except that the polyethyleneporous film in which (C)/(D)=0.08 was replaced as a base material with apolyethylene porous film having the average pore diameter (C) andporosity (D) shown in Table 1 below, where (C)/(D)=0.13, to produce alaminated body including (i) a polyethylene porous film and (ii) aporous layer on a surface of the polyethylene porous film, the porouslayer containing a PVDF-HEP copolymer.

Comparative Example 1

An operation was conducted as in Example 1 except that the polyethyleneporous film in which (C)/(D)=0.08 was replaced as a base material with apolyethylene porous film having the average pore diameter (C) andporosity (D) shown in Table 1 below, where (C)/(D)=0.18, to produce alaminated body including (i) a polyethylene porous film and (ii) aporous layer on a surface of the polyethylene porous film, the porouslayer containing a PVDF-HEP copolymer.

Comparative Example 2

An operation was conducted as in Example 1 except that the PVDF-HEPcopolymer was replaced with carboxymethyl cellulose (CMC) to produce alaminated body including (i) a polyethylene porous film and (ii) aporous layer on a surface of the polyethylene porous film, the porouslayer containing CMC.

Comparative Example 3

An operation was conducted as in Example 1 except that the PVDF-HEPcopolymer was replaced with CHEMIPEARL S300, a polyolefin aqueousdispersion available from Mitsui Chemicals, Inc. to produce a laminatedbody including (i) a polyethylene porous film and (ii) a porous layer ona surface of the polyethylene porous film, the porous layer containingCHEMIPEARL S300, a polyolefin aqueous dispersion available from MitsuiChemicals, Inc.

Comparative Example 4

An operation was conducted as in Example 1 except that the PVDF-HEPcopolymer was replaced with CHEMIPEARL S600, a polyolefin aqueousdispersion available from Mitsui Chemicals, Inc. to produce a laminatedbody including (i) a polyethylene porous film and (ii) a porous layer ona surface of the polyethylene porous film, the porous layer containingCHEMIPEARL S600, a polyolefin aqueous dispersion available from MitsuiChemicals, Inc.

Comparative Example 5

An operation was conducted as in Example 1 except that the polyethyleneporous film in which (C)/(D)=0.08 was replaced as a base material with apolyethylene porous film having the average pore diameter (C) andporosity (D) shown in Table 1 below, where (C)/(D)=0.16, to produce alaminated body including (i) a polyethylene porous film and (ii) aporous layer on a surface of the polyethylene porous film, the porouslayer containing a. PVDF-HEP copolymer.

(Critical Surface Tension Test)

For each of the laminated bodies produced in Examples 1 to 6 andComparative Examples 1 to 5, the measurement was made at 25° C. ofrespective contact angles θ of pure water and propylene carbonate assolvents with respect to the outermost surface of the porous layer(coating layer) of the laminated body. The measurement involved use ofDrop Master 500, a contact angle measuring device available from KyowaInterface Science Co., Ltd. The critical surface tension (A) wascalculated on the basis of a Zisman plot of the measurement results.Another calculation was made through a similar method of the criticalsurface tension (B) over a surface of the porous film (base material)remaining after the porous layer (coating layer) had been peeled fromthe laminated body from which surface the porous layer (coating layer)had been peeled.

(Electrolyte Solution Permeation Test)

For each of the laminated bodies produced in Examples 1 to 4 andComparative Examples 1 to 5, the laminated body was, as illustrated inFIG. 2, fixed to a glass plate with use of a double-sided tape with theporous layer at the top, and 2 μl of an electrolyte solution (diethylcarbonate (DEC)) was dropped onto the porous layer with use of a pipetat a dew point of −20° C. Then, a time period from the drop todisappearance of gloss over the surface of the liquid dropped wasmeasured as a time period (hereinafter referred to as “time period ofpermeation”) necessary for the electrolyte solution dropped to permeatethrough the inside of the laminated body.

(Measurement Results)

Table 1 shows, for each of the laminated bodies produced in Examples 1to 4 and Comparative Examples 1 to 5, (i) the average pore diameter (C)and porosity (D) of the base material (porous film) used, (ii) theaverage pore diameter (C)/porosity (D), (iii) the critical surfacetension (A) and critical surface tension (B) of the laminated bodyproduced, (iv) the difference therebetween ((A)−(B)), and (v) theresults of measurement of the time period of permeation.

TABLE 1 Properties of the base material (porous film) and laminatedbody, and the results of the permeation test Base material (porous film)Average pore Critical surface tension γe (mN/m) Permeation test Averagepore Porosity diameter (C)/ Coating Base material (B) after DifferenceTime period (s) diameter (C) (μm) (D) porosity (D) layer (A) peel ofcoating layer (A) − (B) of permeation Example 1 0.035 0.44 0.08 40 27.812.2 21 Example 2 0.035 0.44 0.08 40.2 35.3 4.9 21 Example 3 0.035 0.440.08 29.7 14.3 15.4 22 Example 4 0.040 0.30 0.13 39 20.1 18.9 28Comparative 0.096 0.54 0.18 40.2 15.4 24.8 57 Example 1 Comparative0.035 0.44 0.08 14.1 15.9 −1.8 297 Example 2 Comparative 0.035 0.44 0.0821.9 25.8 −8.9 140 Example 3 Comparative 0.035 0.44 0.08 20.4 24.1 −3.7380 Example 4 Comparative 0.063 0.39 0.16 38.8 16.3 22.5 51 Example 5

The laminated bodies produced in Examples 1 to 4 each satisfy therelation represented by the following Formula (1):

0 mN/nm≦(A)−(B)≦20 mN/m  (1)

The laminated bodies produced in Comparative Examples 1 to 5, on theother hand, each fail to satisfy the relation represented by Formula (1)above.

The laminated bodies produced in Examples 1 to 4 and ComparativeExamples 2 to 4 each satisfy the relation represented by the followingFormula (2):

(C)/(D)≦0.13  (2)

The laminated bodies produced in Example 6 and Comparative Examples 1and 5, on the other hand, each fail to satisfy the relation representedby Formula (2) above.

Comparison between (i) the time period of permeation for each of thelaminated bodies produced in Examples 1 to 4 and (ii) the time period ofpermeation for each of the laminated bodies produced in ComparativeExamples 1 to 5 shows that the time period of permeation is shorter foreach of the laminated bodies produced in Examples 1 to 4. This indicatesthat a laminated body that satisfies Formula (1) above is permeated moreeasily by an electrolyte solution than a laminated body that fails tosatisfy Formula (1) above.

Comparison between (i) the time period of permeation for the laminatedbody produced in Example 1 and (ii) the time period of permeation foreach of the laminated bodies produced in Example 4 and ComparativeExamples 1 and 5, in each of which the porous film had (C)/(D) differentfrom that of the porous film in Example 1 and in each of which theporous layer included a material identical to that of the porous layerin Example 1 indicates that in a case where the average pore diameter(C)/porosity (D) of the porous film has a smaller value, the time periodof permeation is shorter, and the permeation of an electrolyte solutionis easier.

Examples of Aspect 2 of Present Invention Average Pore Diameter (C) ofPorous Film

The average pore diameter (C) was measured with use of a palm porometeravailable from PMI Co., Ltd. (model: CFP-1500A). The measurementinvolved, as a test liquid, GalWick (product name) available from PMICo., Ltd. and was made of the following curves (i) and (ii) for theporous film:

(i) Pressure-flow rate curve for the porous film as immersed in the testliquid

(ii) Pressure-flow rate curve, which is half the flow rate measured forthe dry porous film

The average pore diameter (C) of the porous film was calculated byFormula (3) below on the basis of the value of a pressure Pcorresponding to the point of intersection of the curves (i) and (ii).

(C)=4 cos θr/P  (3)

In Formula (3) above, (C) represents the average pore diameter (μm), rrepresents the surface tension (N/m) of the test liquid, P representsthe above-mentioned pressure (Pa) corresponding to the point ofintersection, and θ represents the angle (°) of contact between theporous film and the test liquid.

(Porosity (D) of Porous Film)

A square piece with a 10 cm side was cut out from the porous film. Theweight W (g) and thickness E (cm) of the piece cut out were thenmeasured. The porosity (D) of the porous film was calculated by Formula(4) below on the basis of (i) the weight (W) and thickness (E) measuredabove and (ii) the true specific gravity ρ (g/cm³) of the porous film.

Porosity (D)=1−{(W/ρ)}/(10×10×E)  (4)

Production of Separator Example 5

A fully aromatic polyamide (aramid resin) was dissolved inN-methylpyrrolidone at a concentration of 7% by weight, and aluminaparticles were dispersed therein to prepare a coating solution. Thiscoating solution was applied to one surface of a polyethylene porousfilm (base material) having a thickness of 17 μm and the average porediameter (C) and porosity (D) shown in Table 2 below, where(C)/(D)=0.08. The polyethylene porous film was then immersed inisopropyl alcohol to deposit a porous layer containing a resin in whichalumina particles were dispersed in an aramid resin matrix. Thepolyethylene porous film, on a surface of which the porous layer wasdeposited, was then dried to produce a laminated body including (i) apolyethylene porous film and (ii) a porous layer on a surface of thepolyethylene porous film, the porous layer containing a resin in whichalumina particles were dispersed in an aramid resin matrix.

Example 6

An operation was conducted as in Example 5 except that the aramid resinand the alumina particle were replaced with a polyvinylidenefluoride-based resin (vinylidene fluoride-hexafluoropropylene (PVDF-HEP)copolymer) to produce a laminated body including (i) a polyethyleneporous film and (ii) a porous layer on a surface of the polyethyleneporous film, the porous layer containing a polyvinylidene fluoride-basedresin (PVDF-HEP copolymer).

Example 7

An operation was conducted as in Example 6 except that the polyethyleneporous film in which (C)/(D)=0.08 was replaced as a base material with apolyethylene porous film having the average pore diameter (C) andporosity (D) shown in Table 2 below, where (C)/(D)=0.13, to produce alaminated body including (i) a polyethylene porous film and (ii) aporous layer on a surface of the polyethylene porous film, the porouslayer containing a polyvinylidene fluoride-based resin (PVDF-HEPcopolymer).

Comparative Example 6

An operation was conducted as in Example 6 except that the polyethyleneporous film in which (C)/(D)=0.08 was replaced as a base material with apolyethylene porous film having the average pore diameter (C) andporosity (D) shown in Table 2 below, where (C)/(D)=0.16, to produce alaminated body including (i) a polyethylene porous film and (ii) aporous layer on a surface of the polyethylene porous film, the porouslayer containing a polyvinylidene fluoride-based resin (PVDF-HEPcopolymer).

Comparative Example 7

An operation was conducted as in Example 6 except that the polyethyleneporous film in which (C)/(D)=0.08 was replaced as a base material with apolyethylene porous film having the average pore diameter (C) andporosity (D) shown in Table 2 below, where (C)/(D)=0.18, to produce alaminated body including (i) a polyethylene porous film and (ii) aporous layer on a surface of the polyethylene porous film, the porouslayer containing a polyvinylidene fluoride-based resin (PVDF-HEPcopolymer).

Comparative Example 8

First, 25 parts by weight of alumina (AKP3000 available from SumitomoChemical Co., Ltd.) was added to 100 parts by weight of a sodiumcarboxymethylcellulose (CMC, Serogen 3H available from Dai-Ichi KogyoSeiyaku Co., Ltd.) solution (CMC concentration: 0.70% by weight)prepared by dissolving CMC in 20% by weight of an ethanol aqueoussolution and mixed therein to prepare a coating solution. This coatingsolution was applied to one surface of a polyethylene porous film (basematerial) having the average pore diameter (C) and porosity (D) shown inTable 2 below, where (C)/(D)=0.18, and was then dried to produce alaminated body including (i) a polyethylene porous film and (ii) aporous layer on a surface of the polyethylene porous film, the porouslayer containing alumina and CMC.

Comparative Example 9

An operation was conducted as in Comparative Example 8 except that thepolyethylene porous film in which (C)/(D)=0.18 was replaced as a basematerial with a polyethylene porous film having the average porediameter (C) and porosity (D) shown in Table 2 below, where(C)/(D)=0.08, to produce a laminated body including (i) a polyethyleneporous film and (ii) a porous layer on a surface of the polyethyleneporous film, the porous layer containing alumina and CMC.

(Dielectric Strength Test)

For each of the laminated bodies produced in Examples 5 to 7 andComparative Examples 6 to 9, the respective dielectric strengths of theporous layer and the porous film were tested with use of IMP3800K, animpulse insulation tester available from Nippon Technart Inc., throughthe following procedure:

(i) Inserted a laminated body as a measurement target between an uppercylinder electrode with a diameter of φ25 mm and a lower cylinderelectrode with a diameter of φ75 mm.

(ii) Stored electric charge in a capacitor inside the device to apply avoltage increasing linearly from 0 V to the laminated body between theupper and lower electrodes electrically connected to the insidecapacitor.

(iii) Continued applying the voltage until a dielectric breakdownoccurred (that is, until a voltage drop was detected), and measured, asa dielectric breakdown voltage, the voltage at which the voltage drophad been detected.

(iv) Plotted dielectric breakdown voltages with respect to the weightper unit area of the resin in the porous layer of the laminated body,and calculated the dielectric strengths from the inclination of astraight line as a result of linear approximation.

(Withstand Voltage Defect Count Determining Test)

For each of the laminated bodies produced in Examples 5 to 7 andComparative Examples 6 to 9, the laminated body was cut into a 13 cm×13cm piece, and a withstand voltage test was conducted on that piece withuse of withstand voltage tester TOS-9201 available from KikusuiElectronics Corp. The withstand voltage test was conducted under thefollowing conditions:

(i) Inserted a laminated body as a measurement target between an uppercylinder electrode with a diameter of φ225 mm and a lower cylinderelectrode with a diameter of φ75 mm.

(ii) Raised the voltage between the electrodes at a voltage rise rate of40 V/s to 800 V and kept the voltage (800 V) for 60 seconds.

(iii) Conducted a withstand voltage test at 25 positions in the samelaminated body through a method similar to the method described in (i)and (ii).

(iv) Photographed the laminated body with use of a digital still cameraafter the withstand voltage test described in (iii).

(v) Inputted data on the photograph taken in (iv) into a personalcomputer and determined the count of withstand voltage defects with useof IMAGEJ, free image analysis software issued by the NationalInstitutes of Health (NIH), to calculate the number of deficientportions.

(Measurement Results)

Table 2 shows, for each of the laminated bodies produced in Examples 5to 7 and Comparative Examples 6 to 9, (i) the average pore diameter (C)and porosity (D) of the base material (porous film) used, (ii) theaverage pore diameter (C)/porosity (D), (iii) the dielectric strength(A) of the porous layer of the laminated body produced, (iv) thedielectric strength (B) of the porous film of the laminated bodyproduced, and (v) the results of measurement of the withstand voltagedefect count determining test.

TABLE 2 Properties of the base material (porous film) and laminatedbody, and the results of the withstand voltage defect count determiningtest Withstand voltage Base material (porous film) Dielectric strengthtest (V · m²/s) test defect Average pore Porosity Average pore diameterDielectric strength (A) of Dielectric strength (B) of Number of diameter(C) (μm) (D) (C)/porosity (D) porous layer porous polyolefin filmdeficiencies Example 5 0.035 0.44 0.08 690 176 3 Example 6 0.035 0.440.08 270 176 5 Example 7 0.040 0.30 0.13 270 175 22 Comparative 0.0630.39 0.16 270 176 36 Example 6 Comparative 0.096 0.54 0.18 270 254 77Example 7 Comparative 0.096 0.54 0.18 21 254 200 Example 8 Comparative0.035 0.44 0.08 21 176 46 Example 9

Comparison between (i) Examples 6 and 7 and (ii) Comparative Examples 6and 7 shows that among laminated bodies with respective dielectricstrengths close to each other, a laminated body with a smaller value of(C)/(D) has fewer deficient portions during a withstand voltage test, inparticular, a laminated body in which (C)/(D) returns 0.13 or less has30 or less deficient portions, which falls within the preferable range.

Comparison between (i) Examples 5 to 7 and (ii) Comparative Examples 7and 8 shows that a laminated body that satisfies Formula (1) below hasfewer deficient portions than a laminated body that fails to satisfyFormula (1).

(A)>(B)  (1)

Comparison between (i) Example 5 and (ii) Examples 6 and 7 shows thatthe laminated body produced in Example 5 and satisfying (A)>2×(B) hasfar fewer deficient portions than the respective laminated bodiesproduced in Examples 6 and 7 and each failing to satisfy (A)>2×(B).

Examples of Aspect 3 of Present Invention

In Examples, Comparative Examples, and Reference Example below, physicalproperties such as moisture-absorption characteristics and curlcharacteristics of a laminated body were measured through the followingmethod.

(1) Moisture-Absorption Characteristics of Laminated body

Water content rate of laminated body:

A laminated body was cut into three square pieces each measuring 8 cm×8cm, which were then allowed to remain, for one day, (i) at roomtemperature and (ii) with a dew point set to 20° C. and −30° C. Then,with use of a trace moisture measurement device (manufactured byMitsubishi Chemical Analytech Co., Ltd.; CA-200, VA-230), the laminatedbody was heated to 150° C. at a flow rate of 200 mL/min under a nitrogenairflow. Then, a water content detected was measured. A proportion ofthe water content to a total weight of the laminated body before theheating at 150° C. was obtained as a water content rate (% by mass).

Water content rate difference:

A water content rate difference was defined as a value obtained bysubtracting the water content rate at a dew point of −30° C. from thewater content rate at a dew point of 20° C.

Water content difference at certain dew point between polyolefin porousfilm (first porous layer) and coating film (second porous layer):

A polyolefin weight or a coating film weight per square meter isintegrated in each of the water content rate of the polyolefin porousfilm and the water content rate of the coating film at a dew point of20° C., so that respective water content rates of the polyolefin porousfilm and of the coating film per square meter were calculated. Then, anabsolute value of a difference between the respective water contentrates was regarded as a water content difference between the polyolefinporous film and the coating film.

(2) Measurement of area of opening sections of surface of second porouslayer, each of which opening sections is pore of 0.5 μm² or more

With use of a scanning electron microscope (manufactured by HitachiHigh-Technologies Corporation, SU1510), a surface of the second porouslayer was observed with a magnification of 2000. Then, with use of freeimage analysis software “IMAGEJ” developed by the National Institutes ofHealth (NIH: National Institutes of Health), the surface was divided ata luminance at which the pores were detectable. Luminance holes werefilled so that all areas inside the pores were detectable as pore areas.Then, all pores, which fall in a measurement range and each of which hasan area of 0.5 μm² or more, were detected, and a total area of the poresthus detected was calculated. Then, a proportion of the pores, each ofwhich has an area of 0.5 μm² or more, with respect to all the area ofthe measurement range was calculated.

(3) Curl Measurement

A laminated body was cut into a square piece measuring 8 cm×8 cm, andthe square piece was allowed to remain, for one day, (i) at roomtemperature and (ii) with a dew point set to −30° C. Then, a height, bywhich end parts were lifted, was measured. An appearance was alsoassessed according to the following criteria. Note that a state Cindicates a state in which the square piece was completely curled.States A and B are preferable, and the state A is most preferable.

A: No end parts were lifted.

B: Although the ends parts were lifted, a large part of the square pieceother than the end parts was not lifted but was flat.

C: Both end parts were so close to each other that the square piece wasrolled in so as to have a tubular form.

Example 8

A PVDF-based resin (manufactured by Arkema Inc.; product name“KYNAR2801”) was stirred and dissolved in N-methyl-2-pyrrolidone(hereinafter also referred to as “NMP”) at 65° C. for 30 minutes so thata solid content was 7% by mass. A resultant solution was applied as acoating solution to a polyethylene porous film (thickness 17 μm,porosity 36%) through a doctor blade method so as to weigh 1.0 g persquare meter of PVDF-based resin in the coating solution. A laminatedbody, which was an applied material obtained, was immersed in 2-propanolwhile the coating film remained wet with NMP, and then was allowed tostand at 25° C. for 5 minutes, so that a deposited porous film (1-i) wasobtained. While the deposited porous film (1-i) thus obtained was in animmersion solvent wet state, the deposited porous film (1-i) was furtherimmersed in another 2-propanol, and then was allowed to stand at 25° C.for 5 minutes, so that a deposited porous film (1-ii) was obtained. Thedeposited porous film (1-ii) thus obtained was dried at 65° C. for 5minutes, so that a coating separator (1) was obtained. The evaluationresults of the coating separator (1) are shown in Table 3. Note that asecond porous layer of the coating separator (1) had a structure inwhich skeletons each having a diameter of 1 μm or less are bonded toeach other in a three-dimensional network.

Example 9

A coating separator (2) was obtained through a method similar to thatemployed in Example 8 except that a PVDF-based resin content per squaremeter in a coating solution was changed to 3.0 g. The evaluation resultsof the coating separator (2) are shown in Table 3. Note that a secondporous layer of the coating separator (2) had a structure in whichskeletons each having a diameter of 1 μm or less are bonded to eachother in a three-dimensional network.

Comparative Example 10

To a mixture of 100 parts by weight of alumina fine particles(manufactured by Sumitomo Chemical Co., Ltd.; product name “AKP3000f”)and 3 parts by weight of carboxymethyl cellulose (manufactured by DaicelFineChem Ltd.; model No. 1110), water was added so that a solid contentwas 29% by mass. With use of a planetary centrifugal mixer “AWATORIRENTARO” (manufactured by Thinky Corporation;®), a resultant mixture wasstirred and mixed twice at 2000 rpm for 30 seconds at room temperature.To a resultant mixture, 14 parts by mass of 2-propanol was added andmixed, so that a coating solution having 28% by mass of solid contentwas obtained. Through a doctor blade method, a resultant coatingsolution was applied to a polyethylene porous film (thickness 17 μm,porosity 36%), which has been subjected to a corona treatment at 20W/(m²/minute), so that a weight sum of alumina fine particles in thecoating solution and the carboxymethyl cellulose was 7.0 g per squaremeter. A laminated body, which was an applied material obtained, wasdried at 65° C. for 5 minutes, so that a deposited porous film (3) wasobtained. The evaluation results of the deposited porous film (3) thusobtained are shown in Table 3, Note that a second porous layer of thedeposited porous film (3) did not have a structure in which skeletonseach having a diameter of 1 μm or less are bonded to each other in athree-dimensional network.

Comparative Example 11

In a mixed solvent of dimethylacetamide and tripropylene glycol mixed ata ratio of 7/3 [WR], a PVDF-based resin (manufactured by Arkena Inc.;product name “KYNAR2801.”) was stirred and dissolved at 65° C. for 30minutes so that a solid content was 7% by mass. A resultant solution wasapplied as a coating solution to a polyethylene porous film (thickness17 μm, porosity 36%) through a doctor blade method so as to weigh 1.0 gper square meter of the PVDF-based resin in the coating solution. Alaminated body, which was an applied material obtained, was immersed,while the coating film remained in a mixed solvent wet state, in amixture of water, dimethylacetamide, and tripropylene glycol mixed at aratio of 57/30/13 [WR]. Then, the laminated body was allowed to stand at25° C. for 5 minutes, so that a deposited porous film (2-i) wasobtained. While the deposited porous film (2-i) thus obtained was in animmersion solvent wet state, the deposited porous film (2-i) was furtherimmersed in another 2-propanol, and was allowed to stand at 25° C. for 5minutes, so that a deposited porous film (2-ii) was obtained. Thedeposited porous film (2-ii) thus obtained was dried at 65° C. for 5minutes, so that at a coating separator (4) was obtained. The evaluationresults of the coating separator (4) thus obtained are shown in Table 3.Note that a second porous layer of the coating separator (4) had astructure in which skeletons each having a diameter of 1 μm or less arebonded to each other in a three-dimensional network.

Comparative Example 12

A coating separator (5) was obtained through a method similar to thatemployed in Comparative Example 11 except that a deposited porous film(2-ii) was dried at 65° C. for 1 hour. The evaluation results of thecoating separator (5) thus obtained are shown in Table 3. Note that asecond porous layer of the coating separator (5) had a structure inwhich skeletons each having a diameter of 1 μm or less are bonded toeach other in a three-dimensional network.

Comparative Example 13

In a mixed solvent of acetone (good solvent), 2-propanol (poor solvent),and water mixed at a ratio of 130/10/5 [WR], a PVDF-based resin(manufactured by Arkema Inc.; product name “KYNAR2801”) was stirred anddissolved at 40° C. for 30 minutes so that a solid content was 7% bymass. A resultant solution was applied as a coating solution to apolyethylene porous film (thickness 17 μm, porosity 36%) through adoctor blade method so as to weigh 1.0 g per square meter of thePVDF-based resin in the coating solution. A laminated body, which was anapplied material obtained, was dried at 25° C. for 5 minutes in a box inwhich a humidity was adjusted to 40%, so that a coating separator (6)was obtained. The evaluation results of the coating separator (6) thusobtained are shown in Table 3.

Reference Example 1

The evaluation results of each of the polyethylene porous films used inExamples 8 and 9 and Comparative Examples 10, 11, and 12 are shown inTable 3.

TABLE 3 Separator water content Water content difference at dewProportion of pores, each of rate (wtppm) Water content point of 20° C.between first porous which has area of 0.5 μm² or dew point dew pointrate difference layer and second porous layer more, with respect tosurface −30° C. 20° C. (wtppm) (mg/m²) of second porous layer CurlExample 8 161 445 284 4  3% A Example 9 253 577 324 2 26% A Comparative926 2632 1706 35 — C Example 10 Comparative 132 4946 4794 46 — C Example11 Comparative 88 2215 2127 16 — B Example 12 Comparative 60 579 519 361% B Example 13 Reference 33 450 417 — — A Example

Examples of Aspect 4 of the Present Invention Production of Separator

Nonaqueous secondary battery separators in accordance with Examples 10through 13 and Comparative Examples 14 through 18 were produced asfollows.

Example 10

A PVDF-based resin (manufactured by Arkema Inc.; product name“KYNAR2801”) was stirred and dissolved in N-methyl-2-pyrrolidone (NMP)at 65° C. for 30 minutes so that a solid content was 7% by mass. Aresultant solution was applied as a coating solution to a polyethyleneporous base material (thickness 12 μm, porosity 0.44 (44% by volume),average pore diameter (C)/porosity (D)=0.08) through a doctor blademethod so as to weigh 1.0 g per square meter of PVDF-based resin in thecoating solution. An applied material obtained was immersed in2-propanol while the coating film remained wet with NMP, and then wasallowed to stand at 25° C. for 5 minutes, so that a laminated porousfilm (1-i) was obtained. While the laminated porous film (1-i) thusobtained was in an immersion solvent wet state, the laminated porousfilm (1-i) was further immersed in another 2-propanol, and then wasallowed to stand at 25° C. for 5 minutes, so that a laminated porousfilm (1-ii) was obtained. The laminated porous film (1-ii) thus obtainedwas dried at 65° C. for 5 minutes, so that a nonaqueous secondarybattery separator in accordance with Example 10 was obtained.

The nonaqueous secondary battery separator in accordance with Example 10was immersed for 24 hours in an electrolyte solution at 25° C. which wasobtained by dissolving LiPF₆ with a concentration of 1.0 mole per literin a mixed solvent of ethyl methyl carbonate, diethyl carbonate, andethylene carbonate in a volume ratio of 50:20:30. After the immersionfor 24 hours, in the porous layer including the PVDF-based resin, theresin having absorbed the electrolyte solution had a volume of 0.8 cm³per square meter of the porous layer.

Example 11

The nonaqueous secondary battery separator in accordance with Example 11was obtained under the same conditions as those for Example 10 exceptthat the porous base material in Example 10 was replaced with adifferent polyethylene porous base material (thickness 9 μm, porosity0.35 (35% by volume), average pore diameter (C)/porosity (D)=0.13).

The nonaqueous secondary battery separator in accordance with Example 11was immersed for 24 hours in an electrolyte solution at 25° C. which wasobtained by dissolving LiPF₆ with a concentration of 1.0 mole per literin a mixed solvent of ethyl, methyl carbonate, diethyl carbonate, andethylene carbonate in a volume ratio of 50:20:30. After the immersionfor 24 hours, in the porous layer including the PVDF-based resin, theresin having absorbed the electrolyte solution had a volume of 0.8 cm³per square meter of the porous layer.

Example 12

A nonaqueous secondary battery separator in accordance with Example 12was produced under the same condition as those for Example 10 exceptthat the PVDF-based resin (manufactured by Arkema Inc.; product name“KYNAR2801”) was replaced with another PVDF-based resin (manufactured byArkema Inc.; product name “KYNAR2500”).

The nonaqueous secondary battery separator in accordance with Example 12was immersed for 24 hours in an electrolyte solution at 25° C. which wasobtained by dissolving LiPF₆ with a concentration of 1.0 mole per literin a mixed solvent of ethyl methyl carbonate, diethyl, carbonate, andethylene carbonate in a volume ratio of 50:20:30. After the immersionfor 24 hours, in the porous layer including the PVDF-based resin, theresin having absorbed the electrolyte solution had a volume of 1.3 cm³per square meter of the porous layer.

Example 13

A nonaqueous secondary battery separator in accordance with Example 13was produced under the same conditions as those for Example 10 exceptthat the PVDF-based resin (manufactured by Arkema Inc.; product name“KYNAR2801”) was replaced with a resin obtained by mixing the PVDF-basedresin (manufactured by Arkema Inc.; product name “KYNAR2801”) andethylene-vinyl acetate copolymer-based resin in a ratio of 70:30.

The nonaqueous secondary battery separator in accordance with Example 13was immersed for 24 hours in an electrolyte solution at 25° C. which wasobtained by dissolving LiPF₆, with a concentration of 1.0 mole per literin a mixed solvent of ethyl methyl carbonate, diethyl carbonate, andethylene carbonate in a volume ratio of 50:20:30. After the immersionfor 24 hours, in the porous layer including the PVDF-based resin and theethylene-vinyl acetate copolymer-based resin, the resin having absorbedthe electrolyte solution had a volume of 4.3 cm³ per square meter of theporous layer.

Comparative Example 14

A nonaqueous secondary battery separator in accordance with Example 14was produced under the same conditions as those for Example 10 exceptthat the porous base material of Example 10 was replaced with adifferent polyethylene porous base material (thickness 16 μm, porosity0.39 (39% by volume), average pore diameter (C)/porosity (D)=0.16).

The nonaqueous secondary battery separator in accordance withComparative Example 14 was immersed for 24 hours in an electrolytesolution at 25° C. which was obtained by dissolving LiPF₆ with aconcentration of 1.0 mole per liter in a mixed solvent of ethyl methylcarbonate, diethyl carbonate, and ethylene carbonate in a volume ratioof 50:20:30. After the immersion for 24 hours, in the porous layerincluding the PVDF-based resin, the resin having absorbed theelectrolyte solution had a volume of 0.8 cm³ per square meter of theporous layer.

Comparative Example 15

A nonaqueous secondary battery separator in accordance with ComparativeExample 15 was produced under the same conditions as those for Example10 except that the porous base material of Example 10 was replaced witha different polyethylene porous film (thickness 17 μm, porosity 0.54(54% by volume), average pore diameter (C)/porosity (D)=0.18).

The non aqueous secondary battery separator in accordance withComparative Example 15 was immersed for 24 hours in an electrolytesolution at 25° C. which was obtained by dissolving LiPF₆ with aconcentration of 1.0 mole per liter in a mixed solvent of ethyl methylcarbonate, diethyl carbonate, and ethylene carbonate in a volume ratioof 50:20:30. After the immersion for 24 hours, in the porous layerincluding the PVDF-based resin, the resin having absorbed theelectrolyte solution had a volume of 0.8 cm³ per square meter of theporous layer.

Comparative Example 16

A nonaqueous secondary battery separator in accordance with Example 16was produced under the same conditions as those for Example 10 exceptthat the PVDF-based resin (manufactured by Arkema Inc.; product name“KYNAR2801”) was replaced with a resin obtained by mixing the PVDF-basedresin (manufactured by Arkema Inc.; product name “KYNAR2801”) andethylene-vinyl acetate copolymer-based resin in a ratio of 55:45.

The nonaqueous secondary battery separator in accordance withComparative Example 16 was immersed for 24 hours in an electrolytesolution at 25° C. which was obtained by dissolving LiPF₆ with aconcentration of 100 mole per liter in a mixed solvent of ethyl methylcarbonate, diethyl carbonate, and ethylene carbonate in a volume ratioof 50:20:30. After the immersion for 24 hours, in the porous layerincluding the PVDF-based resin and the ethylene-vinyl acetatecopolymer-based resin, the resin having absorbed the electrolytesolution had a volume of 6 cm³ per square meter of the porous layer.

Comparative Example 17

A nonaqueous secondary battery separator in accordance with ComparativeExample 17 was produced under the same conditions as those for Example10 except that the PVDF-based resin (manufactured by Arkema Inc.;product name “KYNAR2801”) was replaced with ethylene-vinyl acetatecopolymer-based resin.

The nonaqueous secondary battery separator in accordance withComparative Example 17 was immersed for 24 hours in an electrolytesolution at 25° C. which was obtained by dissolving LiPF₆ with aconcentration of 1.0 mole per liter in a mixed solvent of ethyl methylcarbonate, diethyl carbonate, and ethylene carbonate in a volume ratioof 50:20:30. After the immersion for 24 hours, in the porous layerincluding the ethylene-vinyl acetate copolymer-based resin, the resinhaving absorbed the electrolyte solution had a volume of 12 cm³ persquare meter of the porous layer.

Comparative Example 18

A nonaqueous secondary battery separator in accordance with Example 18was produced under the same conditions as those for Example 1.0 exceptthat solid content concentration of the PVDF-based resin (manufacturedby Arkema Inc.; product name “KYNAR2801”) was set to 0.3% by mass.

The nonaqueous secondary battery separator in accordance withComparative Example 18 was immersed for 24 hours in an electrolytesolution at 25° C. which was obtained by dissolving LiPF₆ with aconcentration of 1.0 mole per liter in a mixture solvent of ethyl methylcarbonate, diethyl carbonate, and ethylene carbonate in a volume ratioof 50:20:30. After the immersion for 24 hours, in the porous layerincluding the PVDF-based resin, the resin having absorbed theelectrolyte solution had a volume of 0.03 cm³ per square meter of theporous layer.

<Production of Nonaqueous Electrolyte Secondary Battery>

Next, using the nonaqueous secondary battery separators in accordancewith Examples 10 through 13 and Comparative Examples 14 through 18 whichwere produced as above, nonaqueous secondary batteries were produced asfollows.

(Cathode)

A commercially available cathode which was produced by applyingLiNi_(0.5)Mn_(0.3)Co_(0.2)O₂/conductive material/PVDF (weight ratio92/5/3) to an aluminum foil was used. The aluminum foil of the cathodewas cut so that a portion of the cathode where a cathode active materiallayer was formed had a size of 40 mm×35 mm and a portion where thecathode active material layer was not formed, with a width of 13 mm,remained around that portion. The cathode active material layer had athickness of 58 μm and density of 2.50 g/cmt³.

(Anode)

A commercially available anode produced by applyinggraphite/styrene-1,3-butadiene copolymer/carboxymethyl cellulose sodium(weight ratio 98/1/1) to a copper foil was used. The copper foil of theanode was cut so that a portion of the anode where an anode activematerial layer was formed had a size of 50 mm×40 mm, and a portion wherethe anode active material layer was not formed, with a width of 13 mm,remained around that portion. The anode active material layer had athickness of 49 μm and density of 1.40 g/cm³.

(Assembly)

In a laminate pouch, the cathode, the nonaqueous secondary batteryseparator, and the anode were laminated (provided) in this order so asto obtain a nonaqueous electrolyte secondary battery member. In thiscase, the cathode and the anode were positioned so that a whole of amain surface of the cathode active material layer of the cathode wasincluded in a range of a main surface (overlapped the main surface) ofthe anode active material layer of the anode.

Subsequently, the nonaqueous electrolyte secondary battery member wasput in a bag made by laminating an aluminum layer and a heat seal layer,and 0.25 mL of a nonaqueous electrolyte solution was poured into thebag. The nonaqueous electrolyte solution was an electrolyte solution at25° C. obtained by dissolving LiPF₆ with a concentration of 1.0 mole perliter in a mixed solvent of ethyl methyl carbonate, diethyl carbonate,and ethylene carbonate in a volume ratio of 50:20:30. The bag washeat-sealed while a pressure inside the bag was reduced, so that anonaqueous secondary battery was produced.

The nonaqueous secondary battery could be produced by the above methodusing the nonaqueous secondary battery separators in accordance withExamples and Comparative Examples other than Comparative Example 18.However, in the case of the nonaqueous secondary battery separator inaccordance with Comparative Example 18, the separator did not adhere tothe electrode, and consequently the nonaqueous secondary battery couldnot be produced. As above, it was confirmed that the nonaqueoussecondary battery separator in accordance with Comparative Example 18,in which in the porous layer after being immersed in the electrolytesolution, the resin having absorbed the electrolyte solution had avolume of less than 0.05 cm³, could not secure adhesiveness to theelectrode. On the other hand, it was confirmed that in the cases of thenonaqueous secondary battery separators in accordance with Examples 10through 13 and Comparative Examples 14 through 17 other than ComparativeExample 18, in the porous layer after being immersed in the electrolytesolution, the resin having absorbed the electrolyte solution had avolume of not less than 0.05 cm³ per square meter of the porous layer,and consequently the nonaqueous secondary battery separators inaccordance with Examples 10 through 13 and Comparative Examples 14through 17 could secure adhesiveness to the electrode.

<Cyclic Test>

A new nonaqueous secondary battery which had not been subjected to anycycle of charge/discharge was subjected to 4 cycles of initialcharge/discharge. Each cycle of the initial charge/discharge wasperformed under conditions that the temperature was 25° C., the voltagerange was 4.1 to 2.7 V, and the current value was 0.2 C (1 C is definedas a value of a current at which a rated capacity based on a dischargecapacity at 1 hour rate is discharged for 1 hour. The same is appliedhereinafter).

Subsequently, an initial battery characteristic maintaining ratio at 55°C. was calculated in accordance with a formula below.

Initial battery characteristic maintaining ratio (%)=(discharge capacityat 20 C/discharge capacity at 0.2 C)×100

Subsequently, the nonaqueous secondary battery was subjected to 100cycles of charge/discharge. Each cycle of the charge/discharge wasperformed under conditions that the temperature was 55° C.,charge/discharge started from a 50% charge state, constant currents werea charge current value of 1.0 C and a discharge current value of 10 C,and a charge/discharge capacity was 4 mAh. Then, a batterycharacteristic maintaining ratio after 100 cycles was calculated inaccordance with a formula below.

Battery characteristic maintaining ratio (%)=(discharge capacity at 20 Cat 100th cycle/discharge capacity at 0.2 C at 100th cycle)×100

The result is shown in Table 4.

TABLE 4 Volume of resin having absorbed electrolyte Average pore Batterysolution per square diameter (C)/ Initial battery characteristic meterof porous layer Porosity (D) characteristic maintaining after beingimmersed in of porous base maintaining ratio (%) electrolyte solution(cm³) material ratio (%) after 100 cycles Example 10 0.8 0.08 73 65Example 11 0.8 0.12 64 54 Example 12 1.3 0.08 76 73 Example 13 4.3 0.0864 62 Comparative 0.8 0.16 46 31 Example 14 Comparative 0.8 0.18 59 40Example 15 Comparative 6 0.08 33 27 Example 16 Comparative 12 0.08 37 31Example 17

As shown in Table, it was confirmed that the nonaqueous secondarybatteries using the nonaqueous secondary battery separators inaccordance with Comparative Examples 16 and 17, in which in the porouslayer after being immersed in the electrolyte solution, the resin havingabsorbed the electrolyte solution per square meter of the porous layerhad a volume of more than 5.00 cm³, had low initial batterycharacteristic maintaining ratio of less than 60% and low batterycharacteristic maintaining ratio after 100 cycles of not more than 40%.Furthermore, it was confirmed that the nonaqueous secondary batteriesusing the nonaqueous secondary battery separators in accordance withComparative Examples 14 and 15 in which the porous base material had anaverage pore diameter (D)/porosity (C) of more than 0.13 had lowerinitial battery characteristic maintaining ratio of less than 37% andlower battery characteristic maintaining ratio after 100 cycles of notmore than 31%.

In contrast, the nonaqueous secondary batteries using the nonaqueoussecondary battery separators in accordance with Examples 10 through 13,in which in the porous layer after being immersed in the electrolytesolution, the resin having absorbed the electrolyte solution had avolume of 0.05 to 5.00 cm³ per square meter of the porous layer and theporous base material had average pore diameter (D)/porosity (C) of notmore than 0.13, had initial battery characteristic maintaining ratio ofnot less than 60% and battery characteristic maintaining ratio after 100cycles of not less than 50%, and thus could subdue a decrease in thecycle characteristics.

INDUSTRIAL APPLICABILITY Aspect 1 of Present Invention

The present invention relates to a laminated body and a nonaqueouselectrolyte secondary battery separator including the laminated body.The nonaqueous electrolyte secondary battery separator of the presentinvention has improved liquid injection easiness for an electrolytesolution during assembly of a nonaqueous electrolyte secondary battery,and shortens the time period of a step of injecting an electrolytesolution into the battery. The nonaqueous electrolyte secondary batteryseparator of the present invention therefore requires a shorter timeperiod for assembly of a battery, and allows a nonaqueous electrolytesecondary battery to be produced with excellent productivity.

Aspect 2 of Present Invention

The porous layer for the present invention and a laminated bodyincluding the porous layer may each be used broadly in the field ofproduction of a nonaqueous electrolyte secondary battery.

Aspect 3 of the Present Invention

The present invention may be used broadly in the field of production ofa nonaqueous electrolyte secondary battery.

Aspect 4 of the Present Invention

The present invention may be used broadly in the field of production ofa nonaqueous secondary battery.

REFERENCE SIGNS LIST

-   -   1 porous layer containing a resin    -   2 porous film containing a polyolefin as a main component    -   3 pipet    -   4 electrolyte solution    -   5 porous layer    -   6 porous film    -   7 double-sided tape    -   8 glass plate

1.-29. (canceled)
 30. A separator for a nonaqueous electrolyte lithiumsecondary battery which separator is disposed between a cathode and ananode both for the nonaqueous electrolyte lithium secondary battery andwhich separator is used in an electrolytic solution containing a solventprepared by mixing cyclic carbonate and chain carbonate at a mass ratiowithin a range of 20:80 to 40:60, the cyclic carbonate being ethylenecarbonate, propylene carbonate, fluoroethylene carbonate,difluoroethylene carbonate, or a mixture of two or more thereof, thechain carbonate being dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, a fluorine substituent thereof, or a mixture of two ormore thereof, the separator comprising: a porous base materialcontaining a polyolefin as a main component; and a porous layerlaminated on at least one surface of the porous base material, theporous layer containing a polyvinylidene fluoride-based resin, theseparator satisfying(C)/(D)≦0.13, where (C) represents an average pore diameter of theporous base material, the average pore diameter (C) having a value in mindicative of a mean value of respective sizes of pores in the porousbase material, and (D) represents a porosity of the porous basematerial, the porosity (D) having a value indicative of a proportion((F)/(E)) of a volume (F) of void in the actual porous base materialwith reference to a volume (E) of the porous base material assumed tohave no void, the polyvinylidene fluoride-based resin accounting for 70%or more by mass of a resin component of the porous layer, thepolyvinylidene fluoride-based resin containing vinylidene fluoride at aproportion of 95 mol % or more in terms of structure units, an appliedamount of the porous layer on one surface of the porous base materialbeing within a range of 0.5 to 1.5 g/m², in the porous layer after beingimmersed for 24 hours in an electrolyte solution having a temperature of25° C. in which electrolyte solution LiPF₆ having a concentration of 1.0mole per liter is dissolved in a mixed solvent containing ethyl methylcarbonate, diethyl carbonate, and ethylene carbonate at a volume ratioof 50:20:30, the resin having absorbed the electrolyte solution having avolume of 0.05 to 5.00 cm³ per square meter of the porous layer, theporous layer after being immersed for 24 hours in the electrolytesolution having a porosity of 0.5 to 55.0% and an average pore diameterof 0.8 to 95.0 nm.
 31. The separator according to claim 30, wherein inthe porous layer after being immersed for 24 hours in the electrolytesolution, the resin having absorbed the electrolyte solution has avolume of 0.25 to 1.50 cm³ per square meter of the porous layer.
 32. Alaminated body, comprising: a separator according to claim 30; and anelectrode sheet.
 33. A method for producing a laminated body accordingto claim 32, the method comprising the step of applying, to the porousbase material or the electrode sheet, a solution in which the resin forthe porous layer is dissolved.
 34. A nonaqueous secondary battery,comprising: a separator according to claim 30; and an electrode sheet.